speculum volume 43 issue 2 1968 [doi 10.2307%2f2855937] bruce s. eastwood -- mediaeval empiricism-...

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Mediaeval Empiricism: The Case of Grosseteste's OpticsAuthor(s): Bruce S. EastwoodReviewed work(s):Source: Speculum, Vol. 43, No. 2 (Apr., 1968), pp. 306-321Published by: Medieval Academy of AmericaStable URL: http://www.jstor.org/stable/2855937 .Accessed: 18/07/2012 11:00

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MEDIAEVAL EMPIRICISM: THE CASE OF GROSSE- TESTE'S OPTICS* BY BRUCE S. EASTWOOD

THE appearance of an experimental methodology of science in Robert Grosse- teste's writings is often remarked upon by historians of science and forms a convenient focal point for demonstration of the creative interplay between science and philosophy in the Middle Ages.' Grosseteste's optics has been cited as a concrete and important example of this new, experimental method.2 Despite some apparent inconsistency in application3 Grosseteste's method in optical research should in general reflect his logic of science. However, the lack of close correlation between his theory and his practice necessitates reconsideration of Grosseteste's concept of experiment. Because his optical works have borne the burden of proof to a large degree in earlier discussion, a closer examination of the optics is in order as a basis for reassessment.

Robert Grosseteste's approach to the study of optics was in the tradition established by Aristotle and given a clear metaphysico-mathematical statement by Alfarabi. In the Hellenistic world a geometrical approach to optics became stan- dard. After Euclid's Optics the tradition was confirmed by such men as Heron, Ptolemy, Heliodoros, Damianos, and Theon. But the geometrical treatment of light could be more than just a method; its metaphysical significance stemmed from the place of mathematics itself in the Neoplatonic outlook.4 When one re- calls the high place accorded to mathematics in Plato's ontology and epistemology, the metaphysical suggestiveness of applying geometry to any science is clear. Following the Pythagorean rather than the Archimedean view of mathematics, Neoplatonists tended to see an ultimate reality in everything falling under the heading of this discipline. So it was that many mediaeval adherents of the emission theory of vision found support for their view in the geometrical description of the field of vision. If one could treat sight geometrically as if rays were emitted from the eye, then some reality must pertain to this notion. Of course, emission theories predate Neoplatonism and geometrical optics; Aristotle discussed their origins in Presocratic thinkers like Pythagoras and Empedocles. But strength was given to the emission theory by the success of geometry as an analyt- ical tool in optics and by Neoplatonic emphasis on the reality of "geometricals." Alkindi's De aspectibus, 7 (9th century) gave reality to a power sent out from the

* The author wishes to thank Ithaca College and the National Science Foundation, under whose support (GS 1389) the final stages of this paper were prepared.

1 See primarily Ludwig Baur, Die Philosophie des Robert Grosseteste, Bischofs von Lincoln (1253) (Miinster, 1917), and A. C. Crombie, Robert Grosseteste and the Origins of Experimental Science (Ox- ford, 1962).

2 Crombie, Grosseteste, pp. 104-134. 3 Ibid., p. 124. 4 On this question generally see Philip Merlan, From Platonism to Neoplatonism, 2nd edition (The

Hague, 1960). 806

Mediaeval Empiricism: The Case of Grosseteste's Optics 307

eye to a visible object,5 and Robert Grosseteste's De iride (ca 1235)6 defended the mathematici who spoke of emitted visual rays.7 Significantly, both authors were Neoplatonists.

The relationship of optics to geometry in Grosseteste's thought lies in a clear tradition established by Alfarabi (10th century), whose interpretation of Islam as well as physics was colored by Neoplatonism. In his De scientiis,8 Alfarabi's sepa- ration of optics from physics is striking. Optics, along with geometry, arithmetic, astronomy, and music,9 is included as a science of doctrine; all are mathematical, demonstrative disciplines. Optics, the science of direct and reflected vision,10 is directly subordinated to geometry."1 Contrary to Aristotle,12 Alfarabi completely divorced geometrical optics from physics; notably he did not consider practical applications of optics, eg., the study of the rainbow, to be sciences of demonstration, or doctrine. Thus, the contents of Aristotle's Meteorology, including a theory of the rainbow, were subordinated to physics.13 The position of optics according to Alfarabi is not among metaphysical studies, but is among the sciences of demonstration. This viewpoint, faithfully reproduced in Gundissalinus's classification of the sciences,14 was current in the thirteenth century and obviously influential on Grosseteste. While he develops no classification of his own, Grosseteste :does speak of the place of optics among the sciences. In relating the study of the rainbow to divisions of the sciences, he distinguishes optics from physics, for, he says, physics provides the quid of the rainbow but optics provides the propter quid.' Therefore optics as geometry, which Grosseteste describes in De lineis,16 is distinct from physical effects of optical laws, as described in De iride.l7 In following Alfarabi, Grosseteste sees optics essentially as geometry and therefore as a science of demonstration, developed from certain a priori principles, pro-

5 Alkindi, Tideus und Pseudo-Euklid, drei optische Werke, edd. A. A. Bjornbo and S. Vogl (Leipzig, 1912), p. 9

6 Dated thus by R. C. Dales, "Robert Grosseteste's Scientific Works," Isis, LII (1961), 402. 7 Die philosophischen Werke des Robert Grosseteste, Bischofs von Lincoln, ed. Ludwig Baur (MUnster,

1912), p. 73. 8 This work was first translated by Gundissalinus at Toledo (ca 1140) and later by Gerard of Cre-

mona, according to D. Salmon, "The Medieval Latin Translations of Alfarabi's Works," The New Scholasticism, xIII (1939), 245-246. Alfarabi's catalogue was cited in England as early as Alfredus Anglicus, according to G. Lacombe, "Alfredus Anglicus in Metheora," Aus der Geisteswelt des Mittelal ters, edd. Lang, Lechner, and Schmaus (MUinster, 1935), p. 467.

9 The metaphysical significance of the subjects of the mediaeval quadrivium is discussed in Merlan, Neoplatonism, pp. 88-95; the inconsistency of Aristotle's tripartition of knowledge with his denial of subsistent mathematicals is discussed on pp. 59-62.

10 Al-Farabi, Catdlogo de las Ciencias, ed. and transl. Angel Gonzalez-Palencia (Madrid, 1953), pp. 149-151.

1 Ibid., pp. 98-99. 12 Aristotle, Posterior Analytics, I, 27, 87a; Opera (Venice: Juntas, 1552), I, f. 191 r. 13 Al-Farabi, Catdlogo, pp. 161-162. 14 Dominicus Gundissalinus, De divisione philosophiae, ed. Ludwig Baur (Miinster, 1903). 16 Baur, Werke, p. 72; cf. Crombie, Grosseteste, pp. 91-96. 16 Baur, Werke, pp. 59-65; cf. LynnThorndike, A History of Magic and Experimental Science (New

York, 1923), II, 443. 17 Baur, Werke, pp. 72-78.

308 Mediaeval Empiricism: The Case of Grosseteste's Optics

ceeding from the general to the specific. And he goes further than Alfarabi by giving light a metaphysical status in De luce.l8

In his light metaphysics, Grosseteste considers the problem of Creation as a problem of mathematical extension of light from a point source in all directions. Through the Neoplatonic view of Creation by emanation, he sees light as forma corporeitatis, a primary object of creation and an agent of creation as well.19 Light, therefore, becomes the means of spatial extension of matter in creating the sphere of the universe and can be dealt with mathematically. In good Neo- platonic fashion, Grosseteste considers light not only ontologically but also epistemologically and thereby lays the groundwork for his approach to geometrical optics. His epistemology follows in spirit the illumination theory of St. Augustine and others.20 Carried to the West in its most influential form by Gundissalinus, the Neoplatonic epistemology of the thirteenth century was based on an Arabic interpretation of the distinction between inferior and superior objects to which the soul, with its facilities of willing and knowing, can turn.21 So we must recognize two parts of the rational soul, the contemplative intellect and the active intellect, concerned respectively with ultimate truth and external, provisional knowledge: one aspect of the soul furnishes sapientia, while the other provides only scientia.22 Where in this scheme of knowledge does Grosseteste place optics? The answer depends upon varied considerations. The mathematical principles of optics he develops from metaphysical bases as will be shown in detail below. Geometrical optics is in se a form of sapientia; no reference to the external world is required for this cognition. The application of optical principles to specific, mundane problems obliges one to introduce inductive reasoning, to base knowledge on physical perception, to strive for scientia. For Grosseteste the illumination theory of knowledge is more technical than for St Augustine, grace aux arabes. While true knowledge depends on the divine light, man normally finds his spiritual "vision" beclouded and is unable to perceive truth directly. So he turns to his physical senses and arduously pursues an indirect method. Only upon occasion, says Grosseteste, does man's intelligence, the superior part of his soul, receive and make available direct illumination. Normally, it is the rational soul, tied to corporeal objects, which provides knowledge, and this ra-

18 Ibid., pp. 51-59; cf. Crombie, Grosseteste, p. 131. Baur, Philosophie, pp. 76-93. Pierre Duhem, Le systeme du monde (Paris, 1915), III, 284-286.

19 The form of corporeality as the basis of three-dimensional extension and prior to all other forms was also held by Avicenna; Etienne Gilson, History of Christian Philosophy in the Middle Ages (New York, 1955), p. 193. Gundissalinus, the transmitter of many of Avicenna's ideas to the West, incorporated the idea in his psychology; J. T. Muckle, ed., "The Treatise De Anima of Dominicus Gundis- salinus," Mediaeval Studies, II (1940), 55. Also, Grosseteste may well have heard of forma corporeitatis at Paris from Philip the Chancellor; L. W. Keeler, "The Dependence of Grosseteste's De anima on the Summa of Philip the Chancellor," The New Scholasticism, xi (1937), 218. Cf. Crombie, Grosseteste, pp. 104-110; Duhem, Systeme, v, 356-358.

20 See Augustine's.De trinitate, XII, 14; De magistro, 12. See generally Etienne Gilson, "Pourquoi Saint Thomas a critique Saint Augustin," Archives d'histoire doctrinale et littraire du moyen dge, I (1926-1927), 5-127.

21 J. Rohmer, "Sur la doctrine franciscaine des deux faces de l'Ame," Archives d'histoire doctrinale et litteraire du moyen dge, ii (1927), 77.

22 Gundissalinus, De anima, X (ed. cit., pp. 86-87).


tional soul only gradually ascends to full perception of truth. Being weighted down by its earthly condition and sensual data, the rational soul may not achieve sapientia; yet fulness of knowledge is possible, if the soul rises above the concrete, incited by sensual perception and attracted by a higher knowledge.23 Ultimately, Grosseteste finds no essential difference between the illumination by which the blessed see God and that by which man knows intelligibles; the only difference is one of degree based on the freedom of the soul; i.e., our senses are required for knowledge only because of original sin.24

To link sensual knowledge, knowledge of the particular, with original sin is hardly to apotheosize that form of cognition. Grosseteste's epistemology classi- fies experimental knowledge in se on a radically different plane from Truth. Only when illumination assists experience is there an accurate intuitive leap from an inductive train of thought to full knowledge of true causes, and the illumination is the essential basis for such knowledge. If this is Grosseteste's view - and it is - then what use can he see in experimental science or an experimental methodology of science? The answer lies in his synthesis of Platonic and Aristotelian tendencies. The commentary of Grosseteste upon Aristotle's Posterior Analytics is the arena wherein the apparent compromise of the two tendencies issues in the ultimate defeat of the profane by the sacred. Grosseteste finds the Posterior Analytics a description of purely human knowledge; he certifies the description only after affixing the imprimatur of divine illumination. Yet he considers the process described by Aristotle to be important because necessary.25 Man usually learns by induction from particulars, and not always does he learn the truth. When truth is attained, Aristotle's account is accurate for the human side of the cognitive act. Induction, followed by deduction on the basis of discovered cause or principles, is made by Grosseteste the methodological foundation of "natural philosophy" (by which he means science of specifics).26 So in optics he explains any specific occurrence, eg., the rainbow, by appeal to the phenomenon itself and by reasonings drawn from observation; his De iride does this. But general principles of optics, first causes in nature, have metaphysical bases and are not empirically derived; this is apparent in his De lineis, angulis, et figuris. Furthermore, whenever possible, metaphysical principles are used to explain specific phenomena, and we should not be surprised at such procedure from one who considers experimental or experiential knowledge to be a second-rate sort of understanding.

23 Gilson, "Saint Augustin critique," pp. 95-96; L. E. Lynch, "The Doctrine of Divine Ideas and Illumination in Robert Grosseteste," Mediaeval Studies, iii (1941), 169; cf. Richard C. Dales, "Robert Grosseteste's Commentarius in Octo Libros Physicorum Aristotelis," Medievalia et Humanistica, xi (1957), 15. The locus classicus is in Divi Roberti Lincolniensis Archiepiscopi Parisiensis ordinis Praedi- catorum in Aristotelis peripatheticorum principis Posteriorum Analeticorum librum (Venice, 1521), I, 14, f. 20v; this is only one of eight early editions (the last in 1552).

24 Gilson, "St. Augustin critiqu6," p. 98. Grosseteste said in De veritate, "Veritas ... creata ostendit id, quod est, sed non in suo lumine, sed in luce veritatis summae, sicut color ostendit corpus, sed non nisi in luce superfusa" (Baur, Werke, p. 137); and again, ". .. lux summae veritatis et non aliud ostendit mentis oculo id quod est .. ." (ibid., p. 133). Cf. Duhem, Systeme, v, 341-342.

25 Grosseteste, Aristotelis Posteriorum, I, 2 (ed. cit., f. 4r). 28 A full discussion of Grosseteste's methodology can be found in Crombie, Grosseteste, ch. IV. How-

ever, it should be noted that the burden of Crombie's argument is different from the present paper's.


In so far as induction and deduction may give a method for science, this method can be improved by the addition of experimental verification or falsification of any cause discerned. Grosseteste's use of this additional process has been carefully documented by Crombie.27 But we must be careful in defining "experiment." One of the best examples of Grosseteste's use of verification and falsification by experiment appears in his commentary on the Posterior Analytics28 and concerns the purgation of red bile by scammony. As Grosseteste tells it, the observer first notices only the coincidence of the ingestion of scammony with the purgation of red bile. Repeated occurrences suggest a causal connection, for which the observer begins to search. Upon many administrations of scammony alone, he can ex- clude possible but incorrect views of the cause of purgation and can asssure himself that scammony is the true cause of the purgation of red bile in one who has taken scammony. Grosseteste's account indicates use of the dual process of falsification and verification through controlled experiment. Did he then make such an experiment? It seems very unlikely, for the same account of procedure and example is given by Avicenna in more than one place. He refers to it in the Canon29 and again in more detail in his lesser De anima. In the latter place, Avicenna says that the soul, through the animal faculties, ... acquires empirical premisses, which consist in finding through sense-experience the necessary attribution of a positive or negative predicate to a subject, or in finding a contradictory opposition . . ., or in finding a consequence of a positive or negative conjunc- tion .. .; or in finding a positive or a negative disjunction without contradictory opposition .... This relation is valid not sometimes nor in half the number of cases but always, so that the soul acquiesces in the fact that it is of the nature of this predicate to have such- and-such relation to this subject, or that it is of the nature of this consequence to follow necessarily from this antecedent or to be essentially contrary to it-not by mere chance. Thus, this would be a belief obtained from sense-experience and from reasoning as well: from sense-experience, because it is observed; from reasoning, because if it were by chance it would not be found always or even in most cases. It is just as we judge that scammony is, by its nature, a laxative for bile, for we have experienced this often and then reasoned that if it were not owing to the nature of scammony but only by chance, this would happen only on certain occasions.30 Here Avicenna gives the scammony example in detail and discusses both verification and falsification, though he applies only the former in the example. This information was available to Grosseteste through the logic of Algazel, which is a paraphrase of Avicenna's logic;31 the purgation of "bilem flavam" by scammony was mentioned by Galen as well.32

27 Ibid. 28 Grosseteste, Aristotelis Posteriorum, I, 14 (ed. cit., f. 21r); the pertinent passage is translated in

Crombie, Grosseteste, pp. 73-74. 29 Avicenna, Canon, I, ii, 2. 80 Avicenna, Avicenna's Psychology, translated by F. Rahman (Oxford, 1952), p. 55. 81 Julius Weinberg, Abstraction, Relation, and Induction (Madison, Wis., 1965), p. 133. Cf. F. Ueber-

weg, Die patristische und scholastische Philosophie, ed. B. Geyer (Berlin, 1927), p. 311; C. Prantl, Ge- schichte der Logik im Abendlande (Leipzig, 1885), ii, 368.

82 Galen, "Ad Pisonem de Theriaca liber," Claudii Galeni Opera Omnia, ed., Karl Gottlob Kuhn (Leipzig, 1827), xiv, 223.


That Grosseteste's only full discussion of verification and falsification should be modelled on a passage from Avicenna is suggestive; it suggests that the method was theoretical only and not carefully applied in scientific investigations. Such a view fits well into Grosseteste's overall epistemological scheme. In his theoretical method of knowledge, he includes the experimental procedure to in- sure the best possible use of concrete data in leading to knowledge of causes; but he reminds us elsewhere that there are better ways to knowledge. It seems noteworthy that Grosseteste, despite his penchant for optical examples, does not in his commentary on the Posterior Analytics apply to optics the method of experimental verification and falsification.

In order to see just how the inductive-deductive method, including falsification and verification, is used as a scientific tool, we must turn to a detailed analysis of Grosseteste's scientific works. The crucial works are the optical studies, for these represent his mature scientific investigations,33 his major scientific interest, and the branch of science most susceptible to an anti-experimental approach, if we recall the significance of light in Neoplatonic metaphysics.

The interrelationship of the three optical treatises is important. Each develops in a different manner. De lineis, angulis, et figuris, seu de fractionibus et reflexionibus radiorum discusses geometrical optics in the spirit of Alfarabi's classification: in De lineis, Grosseteste lays down the rules of geometrical optics on non-empirical bases. Where references to experience appear, they are given only as examples of the principles concerned. Grosseteste discusses four types of light: direct, reflected, refracted, and accidental. Beyond this he deals with the optical pyramid, the cone formed by rays emanating from a light source, as the normal means of light's transmission in nature. The subsequent treatise, De natura locorum, initially states the essence of De lineis and then directs the reader's attention to applications of the rules to specific natural phenomena. The second treatise is most probably the latter part of a single treatise of which the first part was De lineis.34 In this second part (or treatise, if you wish), Grosseteste deals with examples of direct, reflected, refracted, and accidental light.

It is implicit throughout and occasionally explicit that the natural effects of light occur via optical pyramids. There is in De natura locorum no proper use made of the inductive-deductive method (termed resolution-composition by Grosseteste). The author simply tells the reader which type of light, with its con- comitant rules, lies behind a phenomenon. However, other considerations, i.e., reason and experience, are also brought to bear in order to provide a complete

33 Written from ca 1230 to ca 1235 according to Dales, "Scientific Works," 402. Cf. Thorndike, Magic and Science, ii, 438.

34 De natura locorun is probably a continuation of De lineis. Dales, "Scientific Works," 402, dates both in 1231 and feels this dating to be fairly certain. S. Harrison Thomson, The Writing of Robert Grosseteste (Cambridge, 1940), p. 108, suggests that they may well have originated as a single treatise, for two of the best manuscripts have in common the last sentence of De lineis and the first of De natura locorum, and in the two oldest manuscripts (ca 1275) the two works were copied and rubricated as one (cf. Baur, Werke, pp. 81'-82*). Tnternal evidence also supports this view; the whole first paragraph of De natura locorum is most reasonably viewed as a direct continuation of De lineis, and references to De lineis are made as if to an earlier part of a single treatise.


explanation; thus we find some use of falsification and verification as means to qualify expectations deduced from the optical rules alone.

Only in the third treatise, De iride, seu de iride et speculo, do we find nice accord between practice and theory: in De iride, methodical resolution and composition are backed up by experimental verification and falsification in order to un- cover the cause of the rainbow.35 The resolutio (inductive portion) determines refraction as the cause, while the compositio (deductive portion) demonstrates how refraction of the sun's light is responsible for the rainbow. At various points in the procedure, appeal to experience supports or undermines alternate theories. The argument of De iride differs in form from the previous treatises because of the nature of the subject. Like Alfarabi, Grosseteste sees optics as a study of the geometrical principles of light; but the study of the rainbow is a subdivision of physics and so requires inductive method, appeals to experience, and final verification or falsification through experiment in order to reach tenable conclusions about causation.

To substantiate and elaborate the above conclusions about Grosseteste's optics - his metaphysical and methodological preconceptions - we shall analyze the works in detail. This will lead to a clearer picture of his concept of experiment - what constitutes experiment and when it is to be used. The analysis of the optics will deal with Grosseteste's major points and the degree of experimental basis for each. The method of analysis will be an annotated paraphrase of the optical works. The paraphrase is abbreviated, because some subsidiary points and further examples given by Grosseteste are excluded, for they shed no more light on the point at issue. Grosseteste's sources and my own explications of some of his statements are given in footnotes.

DE LINEIS36 In any discussion of light, we must recognize an active and a passive aspect.

The former relates to light itself, that is, to the nature of light and its mode of generation. The latter relates to the effects of light. When a ray strikes an object, the effect varies with that object, passive in itself, and the variation in effect is due to a variation in the passive object. Thus, a single action, the sun's rays, has various effects depending on the objects affected; the rays dry mud and melt ice.7 To understand the effects of light, we must consider both aspects. The more general of these is the active, for light always acts the same, and its action is according to lines, angles, and figures, i.e., geometrical. Of the possible paths of transition between two points a light ray follows a straight line as the most effective.38 We know this because a straight line is more even, which is better,39

36 Dales, "Scientific Works," 399-401. 36 The text is found in Baur, Werke, pp. 59-65. Cf. Baur, Philosophie, pp. 93-109. 37 Werke, p. 60. This common example is found in various forms in Sextus Empiricus,Adversus

physicos, I, 246-249; Aristotle, Meteorology, II, 5, 362a (ed. Venice: Juntas, 1550, vol. 5, f. 194 v); Gundissalinus, De anima, 4 (":. . unus et idem radius solis diversa agit in diversis, quoniam lutum stringit et ceram dissolvit....").

88 Werke, pp. 60-61. The subsequent supporting statements are Grosseteste's as are all the points made in the text of this analysis.

39 This was available to Grosseteste in Boethius, Arithmetic, I, 32; II, 1.


and because a unified agent, which is best provided by action along a straight line, is the strongest agent.40

When a light ray is reflected, its angles of incidence and reflection must be equal. This is so because of the principle of economy: nature does nothing in vain. In reflection equal angles give the best and shortest path for a ray.41 If reflected, light is weakened,42 and a reflected ray is weaker as it approaches perpendicularity to the reflecting surface.43

Refracted light, on the other hand, is that which passes through a medium rather than being reflected by it. Since refracted light deviates less from a single straight line than does reflected light, refraction causes stronger rays than reflection.44 More specifically, refraction occurs in different ways. A ray passing into a denser medium inclines toward the normal, while the reverse occurs in passage into a rarer medium.45

The fourth type of light, accidental light, is the weakest of all, for it derives only indirectly from an illuminating source. It is this sort of light which enables life to exist on parts of the earth not exposed to the sun.46

No matter which of the above types of light we consider, we must recognize that effects of light generally result from groups of rays rather than single rays. Two figures, or shapes, of the groups are common and natural.47 One is the sphere. In general, all light is projected spherically, that is, in all directions from a point source. Averrois gives us a clear example of this in saying, "... wherever a sense organ is placed, it can feel such an agent at a far distance..."48 The second figure with which we must deal is the pyramid, or cone. When light passes from

40 Aristotle, Metaphysics, V, 6, 1016a (ed. Venice: Juntas, 1552, vol. 8, f. 52 v) is most probably Grosseteste's source for this.

41 Werke, p. 62. Here Grosseteste uses a principle which appears in many places in the optics. He does not explicitly distinguish the metaphysical from the methodological version of this principle, but his use is always of the metaphysical. Here it is used as a reason for and a proof of the equality of angles of incidence and reflection. The principle appears in Aristotle, De caelo, I, 4, 271a (ed. Venice: Juntas, 1550, vol. 5, f. 11 v); that nature should always act in the better way is stated in Plato, Phaedo, 97E-98B.

42 Werke, p. 62. Later Grosseteste supports this by analogy with sound, a common analogy made by Aristotle in De anima, II, 8, 419b (ed. Venice: Juntas, 1550, vol. 6, f. 142 v); Meteorology, III, 4, 374b (ed. Venice: Juntas, 1550, vol. 5, f. 206 r). Macrobius' commentary on the dream of Scipio also states the weakening of light by reflection; v. A. O. Lovejoy, The Great Chain of Being (New York, 1960), p. 63.

43 Werke, p. 63. As Boethius stated (note 39 supra) a straight line is better; so a direct ray is strongest. Therefore, Grosseteste reasons, the more a ray turns from direct progress, the weaker it becomes.

44 Ibid. The same reasoning applies here as for reflection above. 46 Ibid., Both of these directional rules seem to have been available through Alhazen's shorter

treatise on light; v. J. Baarman, transl., "Abhandlung tiber das Licht von Ibn al Haitam," Zeitschrift der Deutschen Morgenldndischen Gesellschaft, XXXVI (1882), p. 224. In any case, these rules were known at the time through the pseudo-Euclidean De speculis, 14; v. Alkindi, Tideus, und Pseudo- Euklid, drei optische Werke, edd., Bj6rnbo and Vogl (Berlin, 1912).

46 Werke, p. 63. Grosseteste's avowed source for this is Albumazar, Introductorium in astronomiam Albumasaris abalachi (Venice, 1506), III, 3. It could also be found in Alhazen's shorter treatise on light (Baarman, "Abhandlung," pp. 214-235).

47 Werke, p. 64. 48 The source here is Averrois' commentary on De anima, II, 7, 419a, in Aristotelis De Anima cum

Averrois Commentariis (Venice, 1579), II, 4, iv, ff. 92r, 93r.


any part of an illuminating agent to the surface of an object, it does so in the shape of a cone.49 Of cone-shaped emissions, one which has its apex nearer to the base is more effective than one with a longer axis.50 The principle of economy shows us that brevity of distance increases the efficiency of the agent. An example of a shorter cone's being more unified and therefore more active is to be found in Euclid's Elements.5'

DE NATURA LOCORUM62

Knowing these geometrical principles of optics, we may now turn to specific natural phenomena to discern their causes in accord with the rules of geometrical optics. The variety of possibilities for the transmission of a ray suggests a variety of effects.

We deal with direct light primarily in terms of the lengths of rays, or cones of rays. While a shorter ray is generally more effective, the rule may be modified by experience and other considerations.53 Evidence for such divergence from reg- ularity is given in mountainous places, which are colder than valleys. But such is only an accidental occurrence: essentially, the reverse should occur. It just so happens that mountains are cold because they extend to a colder region of the atmosphere.54 Again, the equator should be a torrid zone as are the Tropics of Capricorn and Cancer at certain seasons. But the equator is actually a temperate region, and this is known from the evidence of other writers.55 This fact is also substantiated ". . by the theologians who say that Paradise is on the equator in the East."56

49 This basic optical idea appears in many places. Among those available to Grosseteste were Pseudo-Euclid, De speculis, 4; Avicenna, Canon, I, ii, 2; Euclid, Optics, Def. 2, Prop. 23 sqq.

50 Werke, pp. 64-65. 51 Euclid, Elements of Geometry, I 21. Reference to this proposition will show that Grosseteste in-

tends a three-dimensional version of Euclid's construction; he envisions two cones with the same base but with divergent axes so that the apices are neither co-axial nor of equal distance from the common base. The shorter cone Grosseteste considers more "active," or efficacious, partly because its longitu- dinal "sides" (as viewed two-dimensionally) come closer to forming a straight line. But his major point is the principle of economy, that the shortest path is the best and most effective.

62 The text is in Werke, pp. 65-72. 63 Werke, p. 66. Note that the rules of geometrical optics in se are generically correct but may re-

quire modification in specific circumstances; this differentiation parallels and reflects Grosseteste's division of geometrical optics from physics as discussed earlier.

64 Cf. Aristotle, Meteorology, I, 12, 348a (ed. Venice: Juntas, 1550, vol. 5, f. 187 v) for the reason behind this.

66 Werke, p. 66. Grosseteste here refers to Ptolemy, Almagest, II, 6 (Venice, 1515), f. 13v, who reports the view that the equator is temperate; Ptolemy explains that the sun rapidly recedes from the equator and so does not heat it as much as the Tropics. But Ptolemy also says there is no first-hand evidence for the view. Another source for Grosseteste's statement seems to be Avicenna, Canon, I i, 3 (Venice, 1564), f. 12, where support for the view is given; but Canon, I, ii, 2 contradicts Grosseteste's assertion.

66 George Cary, in The Medieval Alexander, ed., D. J. A. Ross (Cambridge, 1956), p. 19, indicates that the notion of Paradise's location in the East stems from a Jewish account as early as A.D. 500 of Alexander's travels up the Ganges; this was incorporated in the Iter ad paradisum, which appears about 1100 in the West. Honorius Inclusus (fl. ca 1090), an English Benedictine (Sarton, Introduction, I, 749), probably based on this Latin source his own placing of Paradise in the far East; see his De


Reflected light, of which there are many interesting examples, is dealt with in terms of cones of rays as well as angles of incidence. The rules established for ascertaining the effects of the sun's rays suggest that the North Pole should be cold and uninhabitable, for the rays would strike obliquely and would approach in longer cones of light, thus being less effective. 57 But experience indicates other- wise, for accidentally these regions are warm in parts. Reflection causes this anomaly.58 Extremely high mountains, such as the Rypheans59 and Hyper- boreans,60 reflect light and bring additional heat to the polar regions. The mountains reflect well, for they contain various reflecting minerals and crystals. Concave reflectors cause excessive heat, while non-concave mountain surfaces reflect enough to sustain a temperate region, e.g., the Hyperboreans.61

Not only geography, but also the seasons find a rationale for climatic variance through the geometry of reflection. On the Tropic of Cancer there is more warmth when the sun's rays come closer to reflection into themselves, the reason being the shorter length of less oblique rays.62 However, summer on the Tropic of Capricorn is warmer than summer on the Tropic of Cancer by the same reasoning. The sun is at its perigee in Capricorn and at apogee in Cancer, thus increasing the brevity of the rays in Capricorn.63 So we can see that even the peculiarities of places, which seem to contradict the geometrical optical rules, can be explained by closer analysis in terms of the laws of geometrical optics.

An especially interesting example of the power of reflected light can be seen in the tides.64 The tide rises as the moon climbs higher in the sky. This is no

imagine mundi, I, 8, published in the opera omnia of Honorius Augustodunensis in Patrologiae cursus completus [latinae], ed. J. P. Migne (Petit-Montrouge, 1854), CLXXII, col. 123; on Honorius Inclusus v. Duhem, Systeme, III, 24-31. Grosseteste's placing of Paradise exactly on the equator probably stems from the fact that medieval cartographers often put Paradise on their maps at the easternmost part, i.e., on the equator; v. C. S. Lewis, The Discarded Image (Cambridge, 1964), p. 144.

67 Werke, p. 68. Grosseteste refers to Aristotle, Meteorology, II, 5, 362a-362b (ed. Venice: Juntas, 1550, vol. 5, ff. 195 r-v), and Ptolemy, Tetrabiblos, II, 3, in support of the coldness of polar regions.

58 Avicenna, Canon, I, ii, 2, notes that mountains in northern countries cause greater warmth by reflection. Aristotle, Meteorology, I, 3,340a (ed. Venice: Juntas, 1550, vol. 5, f. 180 v), says that reflection of the sun's rays causes heat.

69 The Rypheans are identified by Aristotle, Meteorology, I, 13, 350b (ed. Venice: Juntas, 1550, vol. 5, f. 189 r); Pliny, Historia naturalis, IV, 24; and Martinaus Capella, De nuptiis Philologiae et Mer- curii, VI, 663, 665. However, Grosseteste's geography errs in placing these mountains too far north.

60 The Hyperboreans, a fabulous mountain range, were identified by Martianus, De nuptiis, VI, 664, and Pliny, Historia naturalis, IV 26. The latter's colorful report, repeated by Grosseteste (Werke, pp. 68-69), finds the inhabitants of these mountains living an idyllic existence because of the climate: after a long, pleasant life a Hyperborean died, satiated with life, by throwing himself from a high cliff into the sea.

61 Werke, p. 69. 62 Werke, p. 69. Here Grosseteste cites Averrois, Aristotelis De Coelo cum Averrois Commentariis, II,

3, i (vol. 5, Venice, 1579, f. 126r-v). 63 Apogee and perigee are thus given by Ptolemy, Almagest, III, 6. 64 Werke, pp. 69-70. V. Duhem, Systeme, ix, 8-11, 31-33 for comparison of Tidal Theories of

William of Auvergne and Grosseteste.


mere coincidence but a result of an increase in the efficacy of the moon's light as it rises. Again we reason that shorter cones of light rays are more effective.65 The process of tidal rise and fall can therefore be understood as follows. Near the horizon the moon's rays are quite oblique and weak: they have slight influence on the sea. The weak lunar rays form vapors in the sea but ". . . cannot consume them or completely withdraw them to the air."66 That is to say, the sea is expanded by these vapors which do not escape, causing a raising of the sea's surface in the form of a rising tide. As the moon ascends, more and more vapors, or "tumors," are formed in the sea. The increasing strength of the lunar rays then begins to withdraw the vaporous tumors. At its zenith, the moon's rays are strongest, and the tumors completely disappear, allowing the sea to return to low tide. Yet there remains a difficulty. How is the rise and fall of the tide accom- plished simultaneously on opposite sides of the earth? For there is only one moon! Again reflection can be shown to operate. The sphere of the fixed stars provides a reflector, which reflects the moon's rays back to the earth on the opposite side.67

One final example of the importance of reflected rays is the burning mirror. Concave mirrors cause shorter cones of light in reflection and thereby more heat. The great concentration of heat at the focal point of a concave mirror can actually consume a burnable object placed there.68

When we turn to refraction, we find a number of worldly effects of this mecha- nism. Not only the rainbow but other impressiones as well are caused by refraction.69 A good example of the power of refracted rays is given in De proprietatibus

66 Later in De natura locorum (Werke, p. 71) Grosseteste cites the pseudo-Aristotelian De proprietatibus elementorum. That treatise offers a proof, via the Pythagorean theorem, of the point made here in De natura locorum by Grosseteste. The proof assumes a circular orbit of the moon with the earth as a sizeable sphere in the orbit's center; any point (observer) on earth will at one time be closer to the moon, travelling through its orbit, that at other times; v. Vat. ms lat. 2083, f. 210r.

66 Werke, p. 70. 67 Grosseteste refers to Alpetragius' De motibus celorum (probably VII, 12-13, though in confused

fashion) to justify this. Alpetragius actually spoke only of the stellar sphere as an undifferentiated sphere and then of varying obscurities of the stars because of the air. Grosseteste apparently trans- ferred the obscurity of the stars (due to the air) to the stellar sphere itself, thus seeing it as an impene- trable and therefore reflecting surface. He also refers to Messahalla's De scientia motus orbis (Nurem- berg, 1504), n.p., "sermo in scientia magnitudinis solis," which says that the stars draw their light from the sun and so are only reflecting luminaries like the moon. This peculiar idea (that the stars are reflectors only), which Grosseteste was able to put to political use (v. Grosseteste, Epistolae, ed. H. R. Luard, London, 1861, p. 364), appears in various pieces of mediaeval literature, e.g., Isidore of Seville's Etymologiae, III, lxi, and Dante Alighieri's Convivio, II, xiii, 15.

68 Werke, pp. 70-71. Grosseteste cites a "book on mirrors" for this information. This would be either the Euclidean Catoptrics, 30, or the pseudo-Euclidean De speculis, 13 (in the edition of Bjornbo and Vogl).

69 Werke, p. 71. This statement raises a number of questions and is worth a short separate paper by itself. If Grosseteste means refraction alone when he mentions the rainbow, he may already have in mind the theory of De iride; this raises questions of dating. In mentioning other impressiones he is very probably referring only to meteorological phenomena, thus suggesting strongly that the halo is also a result of refraction. Certainly Aristotle discussed both halo and rainbow as meteorology. Did not Roger Bacon give a "Grossetestian" account of the halo? These points I plan to develop in a separate


elementorum.70 If we interject a water-filled glass sphere, e.g., a urinal, between the sun and a burnable object so that solar rays focus on it, that object will be consumed. The explanation of such a phenomenon is in terms of refracted rays. A ray striking the sphere at other than a right angle will be bent towards the perpendicular because of the greater density of the water-filled sphere. The ray then passes through the sphere and is bent away from the perpendicular as it passes out into a rarer medium, the air. In the case of such a body as the urinal, rays will converge on a point which lies along the path of a ray passing through the sphere's center and thus unbent. At this focal point combustion due to the sun's rays can take place.71

Of that most diffuse light, accidental light, one very pertinent example will suffice. Although it is necessary to have a period of the day when sunlight does not shine directly upon us, it is also imperative to have some rays from the sun at all times or life would cease. So at night we receive accidental rays, when the sun is not shining on us, and these rays prevent the air from becoming so con- densed as to kill us.72

DE IRIDE73

Geometrical optics as such is distinct from physics, for the former provides the cause (propter quid) and the latter the fact.74 The study of the rainbow pertains both to optics and to physics; from optics we learn the geometrical rules, while physics describes the specific effects in terms of which the rules must be applied. The present work can be considered an optical treatise, because it deals with causation.75

paper. In any case, Grosseteste's attribution of the rainbow to refraction was completely novel. The only external bases for this notion of his are (1) that the terms refractio andfractio were sometimes used indiscriminately in the early thirteenth century for either reflection or refraction, e.g., by Grosseteste, Werke, pp. 74-75, and (2) that earlier works couple phenomena of reflection and refraction, though un- wittingly, e.g., Seneca, Quaestiones naturales, I, vii, 1-2 (edition of Paul Oltramare, Paris, 1929, vol. 1, p. 33).

70 Werke, p. 71. This treatise exemplifies the primitive trend mentioned in n. 69, the confusion of reflection and refraction, assuming both to be essentially the same. Of the two manuscripts consulted (both having essentially the same description), one (Vat. ms lat. 2083, f. 209r) gives the exact model, a urinal, cited, by Grosseteste in his own text. Grosseteste's geometrical description of the course of the refracted ray through the spherical urinal goes beyond this pseudo-Aristotelian treatise but is the same (not verbatim) as one given in the pseudo-Euclidean De speculis. 14.

71 Other mentions of the burning lens, though with no geometrical description, are in Aristotle, Posterior Analytics, I, 31, 88a (ed. Venice: Juntas, 1552, vol. 1, ff. 193v-194r); Pliny, Historia naturalis, XXXVI, 26; Bartholomew the Englishman, De proprietatibus rerum, VIII, 43.

72 Grosseteste cites a passage from Albumazar, Introductorium in astronomiam Albumasaris abalachi (Venice, 1506), II, 3 (n.p.).

73 Werke, pp. 72-78. Cf. Baur, Philosophie, pp. 109-130. The full title of this work is, of course, De iride seu de iride et speculo. We should note that reflection and refraction are to be studied together (as similar occurrences?). In a manuscript not used by the editor of this treatise (Bibl. Marucell. C. 163, f. 19c), the title is given as Defractionibus radiorum; herefractio may be used in a generic sense to include reflection and refraction,.or, literally, the "bending" of rays in any manner whatsoever.

74 As mentioned above this distinction is found in Alfarabi; v supra. notes 10-11, 13. 76 Werke, p. 72.


Refraction accounts for many amazing phenomena. Magnification by refraction through a spherical lens allows us to view minute objects as if they were large and distant things as if nearby.76 If we look into a bowl and cannot quite see a coin in the bottom because of the rim of the bowl, we can remedy this by pour- ing water into the bowl; now from our original position, we see the coin, for the rays are refracted downward, enabling us to see deeper into the bowl.77

If we wish to determine the exact quantity of the angle of refraction, we can do so as follows. When a ray passes from a less to a more dense medium, its path in the second medium will bisect the angle formed by a perpendicular from the surface and the direct continuation into the second medium of the line it follows in the first.78 This law we learn from the similar law of reflection, which states that the angles of incidence and reflection are equal.79

Just as we find equal angles to be formed in both reflection and refraction, so we observe another similarity. The rule for locating an image in reflection is the same as for refraction. In each case we have a surface (reflecting or refracting), a line drawn perpendicular to the surface from the viewed object, and a ray passing from the eye to the object (reflected or refracted) via the surface. In each case the rule says that the image of the object is located at the intersection of the perpendicular and a rectilinear continuation of the visual ray beyond the point where it strikes the surface of the medium. Here again we learn the application of a rule for reflection to the case of refraction.80 Our reason for these rules of reflection and refraction is "that the whole operation of nature is by the most ordered, shortest, and best means possible."81

The rainbow stands as the most striking of optical phenomena caused by refraction. Previous authors have suggested explanations of the rainbow by other means, but these can be dismissed by simple thought experiments. It would, for instance, be impossible to explain the rainbow in terms of light shining on a

76 Werke, p. 74. Examples of such magnification in the available literature of the time are Euclid, Optics, 2, 5; Seneca, Quaestiones naturales, I, vi, 5. Thorndike, Magic and Science, II, 440, 441.

77 Werke, p. 74. Here Grosseteste cites and quotes directly from Euclid, Catoptrics, def. 6. 78 Werke, p. 74. This peculiar quantitative law of refraction is completely original with Grosseteste.

His reasoning behind the law is discussed in detail in my "Grosseteste's 'Quantitative' Law of Re- fraction: A Chapter in the History of Non-experimental Science," Journal of the History of Ideas, xxvIII (1967), 408-414.

79 Werke, p. 75. For Grosseteste, the equal angles in refraction must be the two halves of the bi- sected angle of incidence, since the angle of incidence is obviously not equal to the angle of refraction.

80 In his rule for the location of the visual image, Grosseteste's analogy was correct; in his attempt to give a quantitative rule for refraction, he was, of course, incorrect. But the reasoning was the same in both cases. The analogy may have been suggested in both cases by Seneca, who assumed certain phenomena of refraction to be ex repercussu and simile speculo in Quaestiones naturales, I, vii, 1-2.

81 Werke, p. 75. Grosseteste applies the principles of economy and uniformity to arrive at the rules for refraction on the basis of reflection. The principle of economy states that nature takes the shortest path. The principle of uniformity states that similar operations will occur in a similar way. In Grosse- teste's mind reflection and refraction are not really different. Both phenomena are effects of a ray striking a surface, and so a related, most economical path must exist for a refracted ray as for a reflected ray. Note the application of the principle of uniformity again, in determining the location of the image in refraction, after using the principle to help derive a quantitative law. Grosseteste had only the laws for reflection in available optical works and neither of the laws for refraction with respect to image location and quantitative angles.


concave cloud, because the rays would not form an arc but rather a shape like the opening in the cloud through which they entered.82 Nor does a concave cloud cause a rainbow by interaction with reflected light; reflection from individual raindrops into the cloud is an incorrect explanation. For raindrops are convex and will reflect the rays principally towards the sun, thus requiring any resultant rainbow to form close to the sun, to rise and fall with it, and to appear at times as a circle and again as an arc. And such a variation in the rainbow accords ill with experience.83

In fact, the rainbow can be successfully explained not by direct or reflected rays but only by refracted rays. Refraction can account for both the shape and the colors of the bow.84 To explain the shape of the bow, we must consider a number of simpler phenomena of refraction, which, when combined, will give a complete picture of the rainbow. The refracting cloud transmits rays in the same manner as does the water-filled glass sphere mentioned above. Then below the cloud the rays will encounter mist or rain. This mist, we say, exists in two regions of differing density, the denser being nearer the earth; and the whole mist is in the shape of a cone descending from the cloud to the earth. Within the mist, then, the rays which have passed from the sun through the spherical cloud will be again refracted, at the surface dividing the rarer from the denser mist. After this last refraction the rays spread forth in the shape of a cone, and the rays representing the surface of such a cone are those responsible for causing the rainbow. Of this cone of rays at least half will always fall upon the earth and have no visible effect; this intersection of the earth with half or more of the cone always results in an arc, never a circle.85 That portion of the cone of rays falling on a cloud causes a rainbow, which is seen by visual rays emitted from the eye.86

82 Grosseteste does not consider the rays to reflect from the cloud to the eye, either here or in his own rainbow theory. His visual theory is an extramission theory, which is evident at many points; v. Werke, p. 73, for his full statement of extramission. In the rainbow he feels that light rays form a visible species on the cloud and that visual rays then proceed from the eye towards this species; sight is created when the visual rays apprehend the species.

83 Werke, pp. 75-76. Grosseteste has here given an explanation of the rainbow by direct light and then has followed it with an explanation by reflection. Since neither accounts for the phenomenon, the only alternative is an explanation by refraction. The polemical procedure here is quite similar to a scholastic quaestio, where "straw-man" alternatives are destroyed largely for effect - to point more emphatically to the author's own solution (v. Crombie, Grosseteste, pp. 87-90). Also, it should be noted that Grosseteste disproves the theories of direct and reflected light by thought experiments only and then says that these theories do not accord with experience: he considers his thought experiment equivalent to experience.

84 Grosseteste's description is given in Werke, pp. 76-77. The text is dense and difficult and has been somewhat simplified, but not misrepresented, in the present paraphrase. A detailed discussion of Grosseteste's rainbow theory appears in my "Robert Grosseteste's Theory of the Rainbow," Arch. Intern. d'Hist, des Sci., LXXVIII (1966), 313-332.

85 In his reflection theory of the rainbow, Aristotle limited the bow to a semicircle as maximum; Meteorology, III, 5, 875b (ed. Venice: Juntas, 1550, vol. 5, f. 207 v).

86 Again it should be stressed that Grosseteste's account of the rainbow is basically an application of simpler phenomena of refraction already known. Also, the speculative nature of his theory is notable. In its development he owes no serious debt to any forerunner in optics. He reduces the possibilities of explanation to refraction alone and then proceeds to use what little is known of refraction to build a rainbow theory. He does not experiment to determine a substantiable theory.


Finally, in explaining the colors of the bow we turn again to refraction and/or reflection.87 Diversity in color is caused by the relative multitude or paucity of light rays. An ordered diminution, caused either by reflection or refraction, will give a regular order of colors.88 This order shows us six colors, each a different combination from the following factors: purity or impurity of the medium, multitude or paucity of light, and clarity or obscurity of light.

CONCLUSIONS The nature of Grosseteste's optical works, especially his use of sources, com-

plements his metaphysical position as outlined above. For Grosseteste geometry represented reality, as a perusal of his De luce will show. In De luce he gives a scientific account of Creation in terms of a geometrical optical atomism, which has strong roots in Plato's Timaeus. Even Aristotle could provide some impetus for mathematical realism, for he stated that there are as many parts of speculative philosophy as there are spheres of being;89 i.e., mathematicals are real. Grosseteste's grouping of optics with geometry is thus a major basis of his light metaphysics; geometry is seen to be as important as light in metaphysics. This relates directly to his style of empiricism in the optics, for there are two aspects of his empiricism. One is epistemology; the other is practical methodology.

Since mundane light is a reflection of metaphysical light, and since both act in accord with geometry, a knowledge of optics is essentially a knowledge of geometry. Therefore optics can be learned by deduction, not requiring inductive reason except in explaining specific physical phenomena caused by light. Since Grosse- teste's epistemology finds direct, deductive knowledge to be preferable to indirect, inductive learning, he should be expected to shy away from an experimental methodology wherever possible. Since he finds metaphysical truths more certain than physical, he applies metaphysical knowledge to optical problems whenever feasible. So he develops the rules of optics like geometrical propositions. Also, he makes notable use at crucial points of the metaphysical principle of economy; he even cites this principle as a proof of the law of reflection. Optics, essentially geometry, can thereby avoid the pitfalls of inductive knowledge, which is only tentative unless confirmed by divine illumination. All this is worked out by Grosseteste in his commentary on Aristotle's Posterior Analytics. The application of such thinking to science is most notable in his optics, because of its geometrical nature. And a detailed analysis of the train of thought in Grosseteste's optical works shows a further pecularity in his concept of empiricism.

Aside from metaphysical limitations, other qualifications to experimental 87 C. .. luminum multiplicatio et a multiplicatione ordinata diminutio non sit, nisi per resplenden-

tiam luminose super speculum, vel a diaphano... ;" Werke, p. 77. 88 Aristotle, Meteorology, III, 4, 373a-373b, 374b (ed. Venice; Juntas, 1550, vol. 15, f. 206r-v), ex-

plains the production of a color, by reflection, which weakens white light by lengthening its path, thus causing color. The pseudo-Aristotelian De coloribus, 1-2 (ed. Venice: Juntas, 1552, vol. 7, f. 76 v) states that the amount of light determines color and discusses variations in light and color according to varying densities of media.

89 Metaphysics, IV, 2, 1004a (ed. Venice: Juntas, 1552, vol. 8, f. 32 r); v. Merlan, Neoplatonism, ch. 3.


knowledge existed for Grosseteste. These expanded the concept of experiment beyond restrictions recognized by modern science. The qualifications of experimentum were as broad as those of "experience," the best translation of the term. Just as Aristotle, Grosseteste appealed to experience, not experiment, in his scientific works. Experience consists of many data not admissible under the heading of experiment; the two realms may be considered opposites in a methodological sense. Experience requires many more presuppositions and is usually cited in support of predetermined theories. Experience is essentially eclectic: one cites examples indiscriminately from memory in support of any given purpose. Experiment, of course, presupposes today the adjective "controlled," and this indicates the difference between experiment and experience. Whether or not experiment itself may be unscientific is another matter. Grosseteste continually cited examples from other writers, which were often incorrect and/or unverifiable. Yet these sources were used with as much certitude as physical experience (not controlled experiment either). Both physical experience and reports by others were given equal credence by Grosseteste. Both were examples of experience, or experimentum.90 And Grosseteste was certainly not alone in such an attitude; it was, in fact, a typical mediaeval tendency, characteristic of an age of authority and of faith. But for Grosseteste especially, the "scientific" methodology of the commentary on the Posterior Analytics was pure epistemology, not a preferred method of practical knowledge in the physical world.

CLARKSON COLLEGE OF TECHNOLOGY

90 For use of experimentum as "experience" in a different framework, v. R6les gascons, IV, edd. Y. Renouard [and P. Chaplais] (Paris, 1962), p. 122, no. 398. Thorndike, Magic and Science, II, 439-440 notes this sense of "experiment" in the work of Grosseteste and cites examples. Cf. the view of Bacon's "experimentalism" in George Sarton, Introduction to the History of Science (Baltimore, 1951), II, 959.

Article Contentsp. 306p. 307p. 308p. 309p. 310p. 311p. 312p. 313p. 314p. 315p. 316p. 317p. 318p. 319p. 320p. 321

Issue Table of ContentsSpeculum, Vol. 43, No. 2 (Apr., 1968), pp. 217-404Front MatterFrench Assemblies and Subsidy in 1321 [pp. 217-244]Dante Musicus: Gothicism, Scholasticism, and Music [pp. 245-257]The Preface to a Fifteenth-Century Concordance [pp. 258-273]Some Mediaeval Moneylenders [pp. 274-289]Class Distinction in Chaucer [pp. 290-305]Mediaeval Empiricism: The Case of Grosseteste's Optics [pp. 306-321]ReviewsReview: untitled [pp. 322-323]Review: untitled [pp. 323-325]Review: untitled [pp. 326-328]Review: untitled [pp. 328-331]Review: untitled [pp. 331-333]Review: untitled [pp. 333-336]Review: untitled [pp. 336-337]Review: untitled [pp. 337-338]Review: untitled [pp. 338-339]Review: untitled [pp. 339-341]Review: untitled [pp. 341-342]Review: untitled [pp. 342-344]Review: untitled [pp. 344-346]Review: untitled [pp. 346-347]Review: untitled [p. 347]Review: untitled [pp. 348-351]Review: untitled [pp. 351-353]Review: untitled [pp. 354-355]Review: untitled [p. 355]Review: untitled [pp. 355-359]Review: untitled [pp. 359-360]Review: untitled [pp. 360-362]Review: untitled [p. 363]Review: untitled [pp. 363-366]Review: untitled [pp. 366-371]Review: untitled [pp. 371-373]Review: untitled [pp. 373-377]Review: untitled [pp. 377-378]Review: untitled [pp. 378-381]Review: untitled [pp. 381-382]Review: untitled [pp. 382-385]Review: untitled [pp. 385-386]Review: untitled [pp. 386-387]Review: untitled [pp. 387-390]Review: untitled [pp. 390-391]Review: untitled [pp. 391-392]Review: untitled [pp. 392-393]Review: untitled [p. 393]Review: untitled [pp. 393-394]

Bibliography of American Periodical Literature [pp. 395-397]Books Received [pp. 398-404]Back Matter

speculum volume 43 issue 2 1968 [doi 10.2307%2f2855937] bruce s. eastwood -- mediaeval empiricism-...

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