Much ink has been spilled in attempting to define the enigmatic category of “enthymeme” that plays such an important role in Aristotle’s rhetorical theory. Aristotle calls enthymemes “the body of proof” (Aristotle, Rhet. 1354a [Freese, LCL]; σῶμα τῆς πίστεως), but nowhere explicitly defines the category.
The typical “textbook definition” tends to try to define enthymemes around either (a) their formal incompleteness in missing one or more premises or (b) their use of more tenuous premises. In his 1959 essay in the Quarterly Journal of Speech, however, Lloyd Bitzer helpfully situates the enthymeme by comparison to other types of syllogisms that Aristotle discusses. Bitzer suggests that
(1) Demonstrative syllogisms are those in which premises are laid down in order to establish scientific conclusions; (2) Dialectical syllogisms are those in which premises are asked for in order to achieve criticism; (3) Rhetorical syllogisms, or enthymemes, are those in which premises are asked for in order to achieve persuasion. (405; underlining added).
Thus, on Bitzer’s reading, the distinguishing features of the enthymeme are not its completeness or incompleteness or the kind of premises it involves. Rather, what sets apart the enthymeme is the manner in which and purpose for which the argument’s premises are obtained.