Deductive Logic

No one knowingly accepts a contradiction, St. George William Joseph Stock argues—yet complicated reasoning and the “infinite perplexities of language” can lead us straight into one.

In this late-19th-century guide, Stock asks what makes thought correct, separating the form of an inference from the subject it concerns. He begins by distinguishing deduction from induction, then tests the foundations of reasoning: identity, contradiction, and the excluded middle. These principles can sound almost too simple to need defending; Stock shows their force by asking what reasoning could mean if a thing might change its identity midway through an argument. Logic, he suggests, is a kind of navigation across thought’s open sea. How do its laws keep us from losing our way?

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