Logic and Reasoning

Type of psychology: Cognition
Fields of study: Cognitive processes; thought
Logic and reasoning are essential elements of the human mind and underlie many
daily activities. Although humans may not follow the prescriptions of formal logic precisely,
human reasoning is nevertheless often systematic. Study of the structures and
processes involved in the use of logic and reasoning provides insight into both the human
mind and the possible creation of intelligent machines.

Key concepts
• atmosphere hypothesis
• availability
• belief-bias effect
• confirmation bias
• deductive reasoning
• gambler’s fallacy
• heuristic
• inductive reasoning
• representativeness
• syllogism

Logical and reasoning tasks are typically classified as either deductive or inductive.
In deductive reasoning, if the premises are true and a valid rule of
inference is used, the conclusion must be true. In inductive reasoning, in
contrast, the conclusion can be false even if the premises are true. In many
cases, deductive reasoning also involves moving from general principles to
specific conclusions, while inductive reasoning involves moving from specific
examples to general conclusions.
Cognitive psychologists study deductive reasoning by examining how
people reason using syllogisms, logical arguments comprising a major and a
minor premise that lead to a conclusion. The premises are assumed to be
true; the validity of the conclusion depends upon whether a proper rule of
inference is used. The classic example of deduction is:
All men are mortal.
Socrates is a man.
Socrates is a mortal.
A more modern example of deduction might be:
All dinosaurs are animals.
All animals are in zoos.
All dinosaurs are in zoos.
The conclusion is valid but is not true, because one of the premises (all
animals are in zoos) is not true. Broadly speaking, truth refers to content
(that is, applicability of the conclusion to the real world), and validity refers
to form (that is, whether the conclusion is drawn logically). It is thus possible
to have a valid argument that is nevertheless untrue. Even if a valid rule
of inference is applied and a valid conclusion is drawn, the conclusion may
not be true. If a valid conclusion has been drawn from true premises, however,
the argument is called “sound.”
With inductive reasoning, the validity of the conclusion is less certain.
The classic example of induction is:
Every crow I have seen in my life up to this time has been black.
All crows are black.
Other examples of induction include a child who begins to say “goed”
(from “go”) instead of “went,” a detective piecing together evidence at the
scene of a crime, and a stock analyst who, after observing that prices have
fallen during the past two Septembers, urges clients to sell in August. In all
these cases, a conclusion is drawn based on evidence observed prior to the
conclusion. There remains the possibility, however, that additional evidence
may render the conclusion incorrect. It does not matter how many positive
instances (for example, black crows, September stock declines) have been
observed; if one counterexample can be found (a white crow, a September
stock rise), the conclusion is incorrect.