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What’s the difference between deduction and induction in an argument?



A deductive inference is a non-probabilistic inference, meaning that, if the premises are true and the reasoning is non-defective, then there is no possibility that the conclusion is false. For example:

The Moon is larger than a golf ball.

Anything larger than a golf ball is larger than an atom.

Therefore, the Moon is larger than an atom.

An inductive inference is a probabilistic inference, meaning that, even if the premises are true and the reasoning is non-defective, there is a chance that the conclusion is false. For example:

Sam’s fingerprints are on the murder weapon.

Nobody besides Sam had access to the murder weapon.

Therefore, Sam is the perpetrator.

Be it noted that the usual definitions of ‘deductive’ and ‘inductive’ are totally false. Some people say that, in deductive inference, ‘there is nothing in the conclusion that is absent from the premises.’ This is totally false. C.f. ‘x is a circle; therefore, x encloses a larger area than any perimeter-equal two-dimensional figure.’ And sometimes it is said that whereas ‘deduction goes from the general to the particular (as in ‘all people are mammals, therefore Smith, being a person, is a mammal’) whereas induction ‘goes from the particular to the general’ (as in, ‘Smith, Jones and Brown are mammals; therefore, all people are mammals). Again, totally false. C.f. Sam was well-disposed towards me before I insulted his cooking and became hostile after I did so, and it was therefore my insulting his cooking that made him ill-disposed towards me.’

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