In formal logic, a syllogism draws a necessary conclusion from certain premises. In rhetoric, such certainty is rarely available and would in any case be inappropriate: an audience cannot verify the premises of a scientific demonstration in real time. The enthymeme draws probable conclusions from probable premises — things that are true for the most part, or that are believed to be true, or that seem plausible to a reasonable person. Probability, not necessity, is the rhetorical standard of proof.
One of the distinctive features of the enthymeme is that it often suppresses a premise — leaving what the audience already accepts as given, so that the argument moves quickly and engages the listener's own participation. When the audience supplies the missing premise themselves, they become active collaborators in the argument rather than passive recipients. This is part of why well-constructed enthymemes are so persuasive: the audience partly convinces itself. Good rhetoric involves knowing exactly which premises can safely be left implicit.
Enthymemes are constructed from topoi — places or lines of argument that can be adapted to many different subjects. Some topoi are specific to a subject matter (legal, deliberative, epideictic); others are general, usable in any domain. The topos of "opposites," for instance, works whenever two contrasting terms allow one to argue from what is true of one to what must be true of the other. Mastery of topoi is the practical core of rhetorical training: the skilled speaker does not invent each argument fresh but draws on a well-organised repertoire of argumentative resources.
The enthymeme is defined and analysed in Rhetoric I.1–2 and II.20–26. Aristotle's treatment of topoi in Rhetoric II.23 lists twenty-eight common lines of argument. The enthymeme concept reappears in Rhetoric's companion work, the Topics, which is devoted entirely to dialectical reasoning from probable premises.
