Induction and Analogy

Induction differs from deduction in that the relation of support between the premises and the conclusion is not intended to be conclusive. The premises are intended only to make the conclusion probably true and, thus, reasonable.

The traditional definition of induction as any argument proceeding from the particular to the general is mistaken.


All the people surveyed thusfar said they preferred brand X.
So, it is likely that the next person surveyed will also claim to prefer brand Z.

Most actors are extroverts.
Jerry is an actor.
So, it is likely that Jerry is an extrovert.

All cows are mammals and have lungs.
All whales are mammals and have lungs.
All humans are mammals and have lungs.
So, it is probable that all mammals have lungs.

Inductive generalization.

Case a was observed to be an F and a G
Case b was observed to be an F and a G
Case c was observed to be an F and a G
So, probably the next F observed will be a G

Case a was observed to be an F and a G
Case b was observed to be an F and a G
Case c was observed to be an F and a G
Nothing was was observed to be an F and not a G
So, probably all Fs are Gs

Size and representativeness of the sample are important factors.

Inductive arguments are not valid or invalid.

Inductives arguments are strong or weak depending on the degree to which the premises support the conlusion.

A strong argument with true premises is said to be cogent.


B is like A in many ways.
A has property G.
So, B has property G.


Principles for evaluating arguments from analogy

1. Relevance of the similarities: The greater the relevance the stronger the argument

2. Number of similarities: The more relevant similarities to better the argument.

3. Nature and degree of disanalogy: The more relevant disanalogies, the weaker the argument

4. Number of primary analogates: the more primary analogates the better the argument

5. Diversity of the primary analogates: The greater the diversity of the primary analogates, the better the argument

6. Specificity of the conclusion: the more specific the conclusion the weaker the argument

Rats are like people in many ways: They have very similar systems of enzymes and hormones, they adapt well to a wide variety of environments, they are omnivores, etc.
People carry umbrellas.
So, rats carry umbrellas, too.

The ways in which A and B are similar should be relevant to having G.

What is the conclusion?

What general principle is illustrated here?

How reasonable is the general principle?

All things that are F are also G
B has F
So B is G

Could also support a weaker principle: Most/many Fs are (likely to be) Gs, or Something that is F is likely to be a G.

General Principle: To be properly called a lover of X a person must love every kind of X.

The Argument from Analogy