Deductive Reasoning

deductive reasoning

the process of drawing logically certain conclusions from general statements or premises.

how deductive reasoning works

• Starts with general facts or rules

• Ends with a sure conclusion

• Based on logic

logical propositions

deductive reasoning is based on

proposition

an assertion, which may be either true or false.

proposition examples

“Cognitive psychology students are brilliant,”
“Cognitive psychology students wear shoes,”
“Cognitive psychology students like peanut butter.”

premises

In a logical argument, ____ are propositions about which arguments are made

logical argument example

Premise 1: If you have a fever, you might be sick.
Premise 2: I have a fever.
Conclusion: I might be sick.

conditional reasoning

One of the primary types of deductive reasoning, in which the reasoner must draw a conclusion based on an if-then proposition.

conditional if-then proposition

if antecedent condition p is met, then consequent event q follows.

modus ponens

the reasoner affirms the antecedent (p). “If p, then q. p. Therefore, q.”

modus tollens

the reasoner denies the consequent (q). "If p, then q. Not q. Therefore, not p.”

you do not have a child

deny the consequent: if you are a mother, then you have a child.

you are a mother

affirm the antecedent: if you are a mother, then you have a child

truth

deductive validity does not equate with ___

p (first phrase)

antecedent condition (what letter?)

example of valid but untrue deduction

“If students eat pizza, then they score high on their exams. They eat pizza. Therefore, they score high on their exams.”

q

consequent event (what letter?)

truth

Deductive Validity does not equate with ____

deductive validity

logical soundness of the reasoning, regardless of truth.

the wason selection task

a logic puzzle used to study how people test if-then statements.

goal of the wason task

test only what logically matters to confirm or deny the rule.

the p (affirming the “if”), and the opposite of q (denying the “then”)

which cards to flip in wason task

modus ponens

People easily recognize and apply this logic (If A, then B; A is true, so B is true).

modus tollens

Harder to understand, therefore, used less (If A, then B; B is false, so A is false).

maybe you passed for another reason

COMMON MISTAKE IN LOGIC: If P, then Q. Q is true → P is true. Example: If you study, you pass. You passed → You must have studied

you might pass without studying

COMMON MISTAKE IN LOGIC: If P, then Q. P is false → Q is false. Example: If you study, you pass. You didn’t study → You didn’t pass.

common mistakes in logic

affirming the consequent and denying the antecedent.

strict logic

We make errors because we rely on how language is usually used in daily life—not ___.

neither is guaranteed

“If you buy this book, you get a rebate. People assume, No book = no rebate, Got rebate = must’ve bought book. But formally,

fail

In abstract tests (like the Wason Selection Task), people often ___.

improves greatly

when the same logic is placed in a real-world context (like checking ID for alcohol), success _______ .

1 and 3

Rule - If drinking beer, must be over 19. Cards - 1.) Drinking beer 2.) Drinking Coke 3.) 16 years old 4.) 22 years old

distrust

Feeling ____ (like seeing an untrustworthy face) makes people think more carefully.

modus tollens

They are more likely to test if something is false using ____, which helps avoid mistakes.