Deduction

Deduction (from Lat. deducere, to take or lead from or out of, derive), a term used in common parlance for the process of taking away from, or subtracting (as in mathematics), and specially for the argumentative process of arriving at a conclusion from evidence, i.e. for any kind of inference.1 In this sense it includes both arguments from particular facts and those from general laws to particular cases. In logic it is generally used in contradiction to “induction” for a kind of mediate inference, in which a conclusion (often itself called the deduction) is regarded as following necessarily under certain fixed laws from premises. This, the most common, form of deduction is the syllogism (q.v.; see also Logic), which consists in taking a general principle and deriving from it facts which are necessarily involved in it. This use of deduction is of comparatively modern origin; it was originally used as the equivalent of Aristotle’s ἀπαγωγή (see Prior Analytics, B xxv.). The modern use of deduction is practically identical with the Aristotelian συλλογισμός.

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1 Two forms of the verb are used, “deduce” and “deduct”; originally synonymous, they are now distinguished, “deduce” being confined to arguments, “deduct” to quantities.

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