Enthymeme. This term in our ordinary logical treatises denotes, and for ages has denoted, a syllogism of which one of the premisses is suppressed in utterance, and the ἐν θυμῷ, which is the main element in the formation of the word, has been absurdly supposed to give it the meaning of something thought but not uttered. Cogito ergo sum is an enthymeme, though its author Descartes did not say it for argumentative ends. The suppressed premiss is quicquid cogitat est. He is contented because he is happy. Here the conclusion needs for its validity a premiss which is not expressed. All the contented are happy.
This is quite different, however, from what Aristotle meant by the enthymeme. In his use of the term it denoted a syllogism from probable propositions or signs. The murderer must have been near at the time. The prisoner was near at the time. Therefore he was the murderer.(1) It is obvious that this is not absolutely conclusive, but a combination of such signs will often be so cogent as to produce rational conviction.
The probable premiss is, as its name imports, no more than probable. The sign may be probable or it may be necessary.(2)
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(1) Archbishop Thomson.
(2) ARISTOTLE, Anal. Pr. II, 27.