A Rulebook for Arguments (Fourth Edition) by Weston Anthony

Author:Weston, Anthony [Weston, Anthony]
Language: eng
Format: mobi
Tags: ebook
Publisher: Hackett Publishing
Published: 2010-04-09T16:00:00+00:00

23 Modus tollens

A second valid deductive form is modus tollens (“the mode of taking”: take q, take p).

If p then q.

Not-q.

Therefore, not-p.

Here “Not-q” simply stands for the denial of q, that is, for the sentence “It is not true that q.” The same is true for “not-p.”

Remember Sherlock Holmes’s argument, discussed under Rule 1:

A dog was kept in the stables, and yet, though someone had been in and had fetched out a horse, [the dog] had not barked. … Obviously the … visitor was someone whom the dog knew well.

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