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Slides Lecture 3. Course: Application of Theories.TRANSCRIPT
Andreas Flache
Manu Muñoz-Herrera
Introduction to formal logicLecture Week 3 - Application of Theories
Block A 2012/2013
http://manumunozh.wix.com/apptheories
Summary Assignment 1
Assignment 1
You were meant to read Lave & March chapters 2 and 3, and Freakonomics chapter 4.
You were supposed to use the 4-steps in the Lave & March model and apply it to 3 explanations given by Levitt and Dubner for the surprising drop in violent crime rates in the US in the 1990s.
You were asked to choose a correct, and incorrect and a surprising (and correct) explanation from the text to do the assignment.
Main observations
The main (and most frequent) limitation in the assignments handed in is that most of them DO NOT contain a discussion of HOW the explanations were tested.
It is important to state (i) what are the implications of the explanation, (ii) how this explanations were tested to account for the implications, and (iii) what was the finding of the authors.
In most case assignments only contained a reference to (iii) and omitted (i) and (ii)
The second main limitation was literal transcription from the book to the assignments. Do not copy but use your own words.
Aims of the lecture
In this lecture we will learn:
How to formulate valid arguments/explanations
How to test whether an argument/explanation is valid
The core methods of so called ``propositional logic’’ and ‘‘syllogistic logic’’
How to generalize and specify concepts and statements
Part 1: How to formulate valid arguments/explanations.
What is logic?Philosophical discipline established by Aristotle
Aristotle384BC - 322BC
Arguments consist of premises and a conclusion
What is logic?Logic is the analysis and appraisal of arguments
An argument is valid means: if all premises are true it is possible that the conclusion is wrong
Premise. If you are reading this, you aren’t illiterate You are reading thisConclusion. You aren’t illiterate
This is wonderful. With the help of logic you can find out whether a statement (conclusion) is true if you know whether other statements (premises) are true.
Thus, if your assumptions are plausible you can make your hypotheses/predictions plausible too (no matter how counter intuitive they are)
What is logic?
No matter how counter intuitive they are???
We distinguish valid from sound arguments.
An argument is valid means: If all premises are true it is impossible that the conclusion is wrong.
Logic provides techniques to test whether a given argument is valid
An argument is sound means: The argument is valid plus all premises are true.
Only if an argument is sound, we can be 100% certain that the conclusion is true. If the argument is valid and at least one premise is false, the
conclusion can be false You need empirical research (and further arguments) to test
whether all premises are true.
Note: Arguments are not true or false (statements are!)
Which argument is valid and which is sound?
If economic welfare increases, the rate of unemployment decreases
In the 1990s, in the US, the economic welfare increased
In the 1990s, in the US, the rate of unemployment decreased
If the rate of unemployment decreases, the rate of violent crimes decreases
In the 1990s, in the US, rate of unemployment decreased
In the 1990s, in the US, the rate of violent crimes decreased
1
2
Basic Propositional Logic
The members of group x are integrated The citizens of Leipzig protest People who hold similar opinions tend to form friendships
Basic propositional logicPropositions are statements, for instance:
Propositions are statements which are either true or false (not valid or invalid)
Shut up! (commands) Why did nobody bring cookies? (question) This is a bad song (normative statements)
Hence, statements which are not true or false are not considered propositions. For instance,
e.g., “I am a sociologist”
Propositional language and truth tables
Propositions are translated into so called “wff’s” (pronounce as woof as in wood). Wff stands for ``well formed formula”
s
Propositions are analyzed using truth tables. Truth tables give a logical diagram for a given wff, listing all possible truth-value combinations.
S
1
0
Symbol of the proposition
Truth values: s can be true (1) or false (0)
Truth-functional operatorsPropositions can be combined, forming new propositions. This is done with so called operators
Operators define the truth-value of the combined proposition based on the truth-values of the propositions that it consists of.
Operator 1: Negatione.g. Assume, s (“I am a sociologist”) is true (1). Then, the negation of s (~s) is false (“I am not a sociologist”).
Symbol: ~ (squiggle) Read: “not”
s ~s
1 0
0 1
If s is true, then the negation is false
If s is false, then the negation is true
Operator 2: Disjunction Symbol: ⋁ (vee) or || or + Read: “or”
p q p⋁q1 1 11 0 10 1 10 0 0
The disjunction of p and q is false if both p and q are false
Operator 3: Conjunction Symbol: ⋅ (dot) or & or ⋀ Read: “and”
p q p ⋅ q1 1 11 0 00 1 00 0 0
The conjunction of p and q is true if both p and q are true
Operator 4: Implication Symbol: ⊃ (horseshoe) or → Read: “if p then q”
p q p⊃q1 1 11 0 00 1 10 0 1
The implication of p and q is false only if p is true and q is false
Example: If Popper is a sociologist, then he is a Marxist.
Popper is a sociologist + Popper is a Marxist : wff is validPopper is a sociologist + Popper is not a Marxist : wff is invalid
Popper is not a sociologist + Popper is a Marxist : wff is valid
Popper is not a sociologist + Popper is not a Marxist : wff is invalid
Operator 5: Equality (biconditional) Symbol: ≡ (threebar) or = or ↔ Read: “if and only if p then q”
p q p≡q1 1 11 0 00 1 00 0 1
The equality of p and q is true if either p and q are both true or both false
Example: If and only if Popper is a sociologist, then he is a Marxist.
Popper is a sociologist + Popper is a Marxist : wff is validPopper is a sociologist + Popper is not a Marxist : wff is invalid
Popper is not a sociologist + Popper is a Marxist : wff is invalid
Popper is not a sociologist + Popper is not a Marxist : wff is valid
Other operators
Exclusive disjunction: true if one but not if both operands are true (XOR, ≠, ⨁) Logical NAND: false if bot operands are true and true if at
least one operand is false (↑,|) Logical NOR: true if both operands are false and false if at
least one operand is true (↓,⊥)
Venn Diagrams
True and False
~p
p
Area inside the circle: possible states where p is true Area outside: possible states where p is false
Disjunction
White area: states where the disjunction of p and q is true Pink area: states where the negation of the disjunction of p and q is true
p q⋁
Conjunction
Pink area: p and not q Blue area: q and not p White area: q and p Purple area: not (q or p)
p qp⋅q
~(p⋁q)
Truth Tables
Working with truth tables
Example: Let us demonstrate for which combination of truth values of p and q is it is correct to state: “p and q are equivalent (p!q)”. Thus, we want to show that:
(if p, then q) and (if q, then p)
p q p⊃qp⊃qp⊃q q⊃pq⊃pq⊃p (p⊃q)·(q⊃p)(p⊃q)·(q⊃p)(p⊃q)·(q⊃p)1 1 1 1 1 1 1 1 1 1 11 0 1 0 0 0 1 1 0 1 00 1 0 1 1 1 0 0 1 0 00 0 0 0 1 0 0 1 1 1 1
Definition of an equality
This proves that: (p!q)!((p⊃q)·(q⊃p))
Rules of Inference
Rules of inference
When we formulate an argument, we infer the conclusion from the premises.
An argument is valid means: If all premises are true it is impossible that the conclusion is wrong.
Thus, if all premises are true, then the conclusion is true.
This is an implication (⊃) In order to show that an argument is valid (that the inference is
correct), we need to demonstrate that the conjunction (·) of all premises implies (⊃) the conclusion.
There are three important forms of argument
Rule 1: Hypothetical Syllogism
Example: If Popper is a sociologist (p), then he is is a Marxist (q)If Popper is a Marxist (q), then he hates capitalism (r)
If Popper is a sociologist (p), then he hates capitalism (r)
General form: p⊃q q⊃r ---------- p⊃r
Demonstrations that the hypothetical syllogism is a valid argument form
Thus, we want to demonstrate that the conjunction (·) of all premises implies (⊃) the conclusion.
Therefore, we need to demonstrate:
if the premises are true, then the conclusion is always true.
This is an implication
This means: (p⊃q)·(q⊃r) This means: (p⊃r)
We need to show that ((p⊃q)·(q⊃r))⊃(p⊃r) is true independent of the truth-values of p, q, and r.
p q r p⊃q q⊃r (p⊃q)·(q⊃r)(p⊃q)·(q⊃r)(p⊃q)·(q⊃r) p⊃r ((p⊃q)·(q⊃r))⊃(p⊃r)((p⊃q)·(q⊃r))⊃(p⊃r)((p⊃q)·(q⊃r))⊃(p⊃r)1 1 1 1 1 1 1 1 1 1 1 11 1 0 1 0 1 0 0 0 0 0 11 0 1 0 1 0 1 0 1 0 1 11 0 0 0 1 0 1 0 0 0 0 10 1 1 1 1 1 1 1 1 1 1 10 1 0 1 0 1 0 0 1 0 1 10 0 1 1 1 1 1 1 1 1 1 10 0 0 1 1 1 1 1 1 1 1 1
The conjunction of the premises logically implies the conclusion. Thus, the hypothetical syllogism is always valid (independent of the truth-values of the truth of the premises)
Is ((p⊃q)·(q⊃r))⊃(p⊃r) always valid?
Rule 2: Modus Ponens
Example: If Popper is a sociologist (p), then he is is a Marxist (q)If Popper is a sociologist (p)
Popper is a Marxist (q)
General form: p⊃qp ----------q
pq
Venn diagram of an implication
Rule 3: Modus Tollens
Example: If Popper is a sociologist (p), then he is is a Marxist (q)If Popper is not a Marxist (~q)
Popper is not a sociologist (~p)
General form: p⊃q~q ----------~p
pq
Venn diagram of an implication
Syllogistic Logic
Syllogistic Logic
Like propositional logic, it is a branch of logic. Propositional logic focuses on propositions which refer to single
objects (i.e., Popper In contrast, syllogistic logic is concerned with domains of objects
Propositional logic: It rains (r) Popper is cool (c)
Syllogistic logic: All swans are white (all S is W) Societies with high anomie suffer
from high crime rates (all A is C)
With syllogistic logic, we study the implications of general statements (laws). Remember that our theories are general statements
Typical wffs from:
Formulating wffs in syllogistic logic
To formulate a correct wff, you need only five words:
all no some is not
Formulating wffs in syllogistic logic
There are only eight (8) forms of wffs:
all A is B All swans are white no A is B There are no white swans some A is B Some swans are white some A is not B Some swans are not white x is B This swan is white x is not B This swan is not white x is y This is the only white swan x is not y This is not the white swan
Any sentence can be translated into a wff of one of these forms
Implications in syllogistic logic
General form of an implication: all A is BRead: For all objects in the domain, if an object is A then it is B
Use capital letters to refer to domains of objects (all)
Use small letters to refer to single objects (me, Popper)
Why is “all A are B” an implication?
(a1⊃b1)·(a2⊃b2)·(a3⊃b3)...(an⊃bn)
Rules of Inference
Rule 1: Hypothetical Syllogism
Example: All sociologists (S) are Marxists (M)All Marxists (M) are against capitalism (C)
All sociologists (S) are against capitalism (C)
General form:all S is Mall M is C ----------All S is C
Venn diagram
SMC
Rule 2: Modus Ponens
Example: All sociologists (S) are Marxists (M)Popper (p) is a sociologist (S) [p is S]
Popper (p) is a Marxist (M) [p is M]
General form:all S is Mp is S ----------p is M
SM
Venn diagram of an implication
★
Popper
Rule 3: Modus Tollens
Example: All sociologists (S) are Marxists (M)Popper (p) is not a Marxist (M) [p is not M]
Popper (p) is not a sociologist (S) [p is not S]
General form:All S is Mp is not M ----------p is not S
SM
Venn diagram of an implication
★
Popper
The Star Test
Testing whether a syllogism is valid: The star test
The star test consist of three steps:
Step 1: Find the “distributed letters”
A letter is distributed if it occurs just after “all” or anywhere after “no” or “not”
all A is Bno A is Bx is Ax is not y
Underline the distributed letters
Testing whether a syllogism is valid: The star test
Step 2: Star premises letters which are distributed and conclusion letters which are not distributed
all A* is Bsome C is A-----------------some C* is B*
Testing whether a syllogism is valid: The star test
Step 3: Decide. A syllogism is valid if and only if every capital letter is starred exactly once.&if there is exactly one star on the right hand side
all A* is Bsome C is A-----------------some C* is B*
Each capital letter is starred exactly once
There is exactly one star at the right hand side (see the B)
Thus, this syllogism is valid.
Second example:
no A* is B*no C* is A*-----------------no C is B
Is it a valid syllogism?
Second example:
no A* is B*no C* is A*-----------------no C is B
A is starred twice.
There are two stars on the right hand side (see A and B)
Thus, there are two reasons why this syllogism is not valid.
Generalizing and Specifying Concepts
Abstract &
Generalize
Specify Classify
Relation between humans
Social relations
Friendships
Friendships between students
Friendships between first-years
dyadsRelation between
humans
Social relations
Friendships
Friendships betweenstudents
Friendships between
first-years
Specify:
Generalize:
Include more characteristics in the definition of the concept
Fewer objects fall under the concept
Abstract more details
More objects fall under the concept
All sociologists (S) are good statisticians (G). (S⊃M)
S=df. Everybody with at least a Doctor’s degree in Sociology
S=df. Everybody with a university degree in Sociology
G=df. Everybody who can interpret a
regression
G=df. Everybody who can explain what a
regression is
1
2
4
3
Generalizing and specifying implications
Generalize the implication: from 1 to 3, or from 2 to 4
Specify the implication: from 3 to 4, or from 1 to 2
The information content of an implicationScientists seek to formulate informative statements. Thus, they should inform us about many things and make precise predictions.
Independent variable (if) should be general (true for many cases)
Dependent variable (then) should be very specific (true for few cases)
S=df. Only modern human societies
S=df. All human societiesD=df. Increase in
complexity
D=df. Increase in stratification
⊃
Societies (S) Differentiate (D)
⊃Modern human
societies
Traditional human
societies
All human societies
Anything can happen
Social differentiation
Social differentiation& conflicts
Implications are more informative if:
You use disjunctions in the if part (if A or B or C)
You use conjunctions in the then part (then X and Y and Z)
Assignment
In the reader you have the second chapter of this book
Read this chapter and do the exercises in the assignment guide in Nestor