Enthymeme

An enthymeme (Aristotle's term, from Greek en thymos, 'in the mind') is a syllogism with one premise omitted. Instead of stating 'all men are mortal; Socrates is a man; therefore Socrates is mortal', the speaker says simply 'Socrates is mortal, because he is a man', and lets the audience supply 'all men are mortal' silently. Aristotle called the enthymeme 'the body of persuasion' — the fundamental unit of practical argument, distinct from the formal syllogism of pure logic.

The rhetorical power of the enthymeme is that the audience does not merely listen to the argument, they help make it. By supplying the missing premise themselves, they are less likely to reject the conclusion, because rejecting it would require them to first identify and then repudiate a premise they have already, silently, granted. Every good political slogan is an enthymeme: 'Yes we can' has an unstated 'because …' that the audience fills in from its own beliefs.

The danger is that the omitted premise may be false, or may not be one the audience would consciously endorse. A great deal of manipulation, in advertising and in politics, works by getting the audience to supply, in the privacy of their own head, a premise the speaker could not defensibly assert aloud. Careful listeners train themselves to reconstruct the missing premise of every enthymeme they encounter.

For writers, the enthymeme is the natural shape of a punchy sentence. Long-form arguments should occasionally lay out the full syllogism, so that the reader can inspect each premise. Short-form persuasion — the headline, the pull quote, the closing line — almost always works better as an enthymeme.