Week 1 – Introduction and Logic
Chapter 1 PS: Introduction
1.1 Why psychology and science?
Psychology aims to study and explain questions of what it is to be human.
Logical Positivism – idea that only statements that can be tested by experience or are logically true are meaningful.
1.2 Trust in psychology as a science
Recent events have caused increased doubt in how psychology can be trusted as a science:
Replication crisis -> many psychological studies fail to produce the same results when replicated. Journals often
avoid publishing replication studies because they attract fewer citations, which hurts their prestige. A major project
attempting to replicate 100 studies found that many could not be replicated and even fewer produced statistically
significant results
Scientific fraud -> in 2011, professor Stapel, a respected academic, was exposed for fabricating data in over 50
publications. Despite collaborating with many co-authors and passing peer review, his deception went undetected
for over two decades. The case highlighted issues like verification bias, where researchers may manipulate or
selectively report data to support a desired hypothesis.
1.3 Overview of the course
Summary of empiricism up to Logical positivism:
• 1600 – Francis Bacon: introduced ‘induction’ – science results from observations, organizing these facts,
and using induction to infer generalization of ‘laws’.
• 1700 – Empiricists (Locke, Hume): knowledge is acquired only through experience
• 1850 – Positivism (Auguste Comte): science should focus only on ‘positive’ observables, no metaphysics
(speculation)
• 1900 – Positivism (Ernst Mach): Start from observable facts, no metaphysics, no theories or models.
• 1920 – Neo-Positivism/Logical Positivism (Vienna Circle – Wiener Kreis, Carnap, Hempel, Neurath, Schlick):
theories and models can be used if they are derived completely from logic and observation.
Chapter 6 PS: Logic
In this chapter we discuss the different forms of logic
6.2 Deductive Reasoning and Syllogisms (Categorical Logic)
Categorical Logic deals statements about categories or groups, like “all
dogs are mammals”.
A syllogism -> two or more premises + one conclusion -> A premises and
a conclusion are statements -> A statement is declarative sentence that is
either true or false.
Validity ≠ truth -> its possible to have a valid deduction but an untrue
conclusion, or an invalid deduction but a true conclusion.
True: in accordance with the world (about statements)
Valid: internally correct deduction (about syllogisms)
Sound: valid deduction based on true premises
Syllogism:
All people with MDD have SD
Or
All people with MDD are people with SD
Or
All MDD are SD
,The above are all statements, as they assert or claim something. We can also write:
Some MDD are SD
Categorical statements/claims are statements with the words ‘all’ and ‘some’, they state whether something
belongs to a specific category or not.
Aristotle identified 4 types of categorical claims:
General Affirmative (A) All MDD are SD
Negative (E) No MDD are SD
Specific Affirmative (I) This MDD is SD (Some MDD are SD)
Negative (o) This MDD is not SD (Some MDD are not SD)
In a syllogism, each statement follows the format
‘subject = predicate’, where the predicate
expresses a characteristic of the subject in the
sentence.
• The major term (P) of the conclusion comes from the predicate of one premise
• The minor term (S) comes from the subject of the other premise
• The middle term (M) appears in both premises but not in the conclusion, yet it links the premises
together
In the Deductive-Nomological (D-N) mode
1. First premise (the law)
è A general rule or law
Example: all metals expand when heated
2. Second premise (the case)
è A specific situation
Example: this object is made of metal
3. Conclusion (the prediction)
è The logical result
Example: this object will expand when heated
6.3 Venn diagrams and syllogisms
Venn diagrams visually represent categorical syllogisms by showing relationships between sets with overlapping
circles. They help test the validity of arguments by checking if conclusions follow logically from the premises.
Syllogism 2 has a valid deduction:
• Premise 1: all MDD are SD -> MDD circle entirely within SD.
• Premise 2: Some W are MDD -> X exists in both W and MDD,
thus also in SD
• Conclusion: Some W are MDD -> valid because the
conclusion necessarily follows.
, Syllogism 3 has an invalid deduction:
• Premise 1: all MDD are SD
• Premise 2: Some W are SD -> X may be in SD with or without
being in MDD
• Conclusion: some W are MDD -> invalid since multiple
possible placements of X create uncertainty.
Syllogism 4 has a valid form but a potentially false conclusion:
• Premise 1: All MDD are SD
• Premise 2: All W are MDD
• Conclusion -> All W are SD -> valid structure, but if any
premise is factually false, so is the conclusion.
A proof must exclude all alternatives to be valid. Logic distinguishes between validity (proper form) and truth
(factual accuracy). Proof by contradiction can be used to test validity.
6.4 Propositional logic and truth tables
Propositional logic deals with entire statements (propositions) that are either true (1) or false (0). It uses logical
connectives to build more complex statements.
Types of propositions:
• Simple propositions: cannot be broken down further (e.g., “it is raining.”)
• Composite propositions: combine simple propositions with logical connectives, like and, or, if…then (e.g.,
“it is raining, and it is cold.”)
Connectives and negation are considered truth-functional operators, their impact isn’t influenced by the content
of the propositions but only by their truth-values (1=true,0=false) when combined.
Modus ponens means that if an “if-then” statement is true, and the “if” part happens, then the “then” part must
also happen. E.g.: If it’s sunny, then we will go outside.
It is sunny -> we will go outside.
1 -> this is modus ponens.
0 -> if a happens but B doesn’t, statement is false.
1 -> if A doesn’t happen, “if A then B” stays true.
1 -> if A doesn’t happen, “if A then B” stays true.
Chapter 1 PS: Introduction
1.1 Why psychology and science?
Psychology aims to study and explain questions of what it is to be human.
Logical Positivism – idea that only statements that can be tested by experience or are logically true are meaningful.
1.2 Trust in psychology as a science
Recent events have caused increased doubt in how psychology can be trusted as a science:
Replication crisis -> many psychological studies fail to produce the same results when replicated. Journals often
avoid publishing replication studies because they attract fewer citations, which hurts their prestige. A major project
attempting to replicate 100 studies found that many could not be replicated and even fewer produced statistically
significant results
Scientific fraud -> in 2011, professor Stapel, a respected academic, was exposed for fabricating data in over 50
publications. Despite collaborating with many co-authors and passing peer review, his deception went undetected
for over two decades. The case highlighted issues like verification bias, where researchers may manipulate or
selectively report data to support a desired hypothesis.
1.3 Overview of the course
Summary of empiricism up to Logical positivism:
• 1600 – Francis Bacon: introduced ‘induction’ – science results from observations, organizing these facts,
and using induction to infer generalization of ‘laws’.
• 1700 – Empiricists (Locke, Hume): knowledge is acquired only through experience
• 1850 – Positivism (Auguste Comte): science should focus only on ‘positive’ observables, no metaphysics
(speculation)
• 1900 – Positivism (Ernst Mach): Start from observable facts, no metaphysics, no theories or models.
• 1920 – Neo-Positivism/Logical Positivism (Vienna Circle – Wiener Kreis, Carnap, Hempel, Neurath, Schlick):
theories and models can be used if they are derived completely from logic and observation.
Chapter 6 PS: Logic
In this chapter we discuss the different forms of logic
6.2 Deductive Reasoning and Syllogisms (Categorical Logic)
Categorical Logic deals statements about categories or groups, like “all
dogs are mammals”.
A syllogism -> two or more premises + one conclusion -> A premises and
a conclusion are statements -> A statement is declarative sentence that is
either true or false.
Validity ≠ truth -> its possible to have a valid deduction but an untrue
conclusion, or an invalid deduction but a true conclusion.
True: in accordance with the world (about statements)
Valid: internally correct deduction (about syllogisms)
Sound: valid deduction based on true premises
Syllogism:
All people with MDD have SD
Or
All people with MDD are people with SD
Or
All MDD are SD
,The above are all statements, as they assert or claim something. We can also write:
Some MDD are SD
Categorical statements/claims are statements with the words ‘all’ and ‘some’, they state whether something
belongs to a specific category or not.
Aristotle identified 4 types of categorical claims:
General Affirmative (A) All MDD are SD
Negative (E) No MDD are SD
Specific Affirmative (I) This MDD is SD (Some MDD are SD)
Negative (o) This MDD is not SD (Some MDD are not SD)
In a syllogism, each statement follows the format
‘subject = predicate’, where the predicate
expresses a characteristic of the subject in the
sentence.
• The major term (P) of the conclusion comes from the predicate of one premise
• The minor term (S) comes from the subject of the other premise
• The middle term (M) appears in both premises but not in the conclusion, yet it links the premises
together
In the Deductive-Nomological (D-N) mode
1. First premise (the law)
è A general rule or law
Example: all metals expand when heated
2. Second premise (the case)
è A specific situation
Example: this object is made of metal
3. Conclusion (the prediction)
è The logical result
Example: this object will expand when heated
6.3 Venn diagrams and syllogisms
Venn diagrams visually represent categorical syllogisms by showing relationships between sets with overlapping
circles. They help test the validity of arguments by checking if conclusions follow logically from the premises.
Syllogism 2 has a valid deduction:
• Premise 1: all MDD are SD -> MDD circle entirely within SD.
• Premise 2: Some W are MDD -> X exists in both W and MDD,
thus also in SD
• Conclusion: Some W are MDD -> valid because the
conclusion necessarily follows.
, Syllogism 3 has an invalid deduction:
• Premise 1: all MDD are SD
• Premise 2: Some W are SD -> X may be in SD with or without
being in MDD
• Conclusion: some W are MDD -> invalid since multiple
possible placements of X create uncertainty.
Syllogism 4 has a valid form but a potentially false conclusion:
• Premise 1: All MDD are SD
• Premise 2: All W are MDD
• Conclusion -> All W are SD -> valid structure, but if any
premise is factually false, so is the conclusion.
A proof must exclude all alternatives to be valid. Logic distinguishes between validity (proper form) and truth
(factual accuracy). Proof by contradiction can be used to test validity.
6.4 Propositional logic and truth tables
Propositional logic deals with entire statements (propositions) that are either true (1) or false (0). It uses logical
connectives to build more complex statements.
Types of propositions:
• Simple propositions: cannot be broken down further (e.g., “it is raining.”)
• Composite propositions: combine simple propositions with logical connectives, like and, or, if…then (e.g.,
“it is raining, and it is cold.”)
Connectives and negation are considered truth-functional operators, their impact isn’t influenced by the content
of the propositions but only by their truth-values (1=true,0=false) when combined.
Modus ponens means that if an “if-then” statement is true, and the “if” part happens, then the “then” part must
also happen. E.g.: If it’s sunny, then we will go outside.
It is sunny -> we will go outside.
1 -> this is modus ponens.
0 -> if a happens but B doesn’t, statement is false.
1 -> if A doesn’t happen, “if A then B” stays true.
1 -> if A doesn’t happen, “if A then B” stays true.
