Enthymeme (Rhetoric)

From Greek: ἐνθύμημα [enthýmema]: thought, argument. A rhetorical stylistic device in which part of an argument (usually a premise) is only implied rather than elaborated.

As such a rhetorical device it is found, for example, in the following statement:

“Three Colours: Blue” is a French film, therefore it is boring.

This hides the implied assertion that “all French films are boring”. A statement that probably not all film lovers would agree with unconditionally.

The specific use of this term has changed over time. Originally, it referred to various forms of persuasive speech. This included, in particular, argumentation which we would today call inductive or abductive. However, already Aristotle emphasised the aspect that in enthymemes well-known and generally accepted premisses may be omitted.

An example of such an argument would be:

Socrates is mortal, because he is a human being.

The premise implied here, “all humans are mortal”, is certainly well-known and can be assumed to stand undisputed.

This also leads to the modern use of this term, which refers specifically to the omission of a part or of parts of the argument.

While this is typically understood as as the omission of premises, it may also refer to an omitted conclusion, as in the following example:

The air pressure has fallen – and when the air pressure falls, this usually means storm.

The conclusion “there will (probably) be a storm” can be omitted here without any problem.

As a rhetorical device, enthymemes are widely used and in fact often very useful: it would certainly be rather tiring for the audience and also for other participants in the discussion if someone insisted in fully explaining every single premise used in an argument. That is, as long as these premisses are understood by everybody and undisputed.

The same practice becomes problematic, however, when enthymemes are used to put forward positions that are not generally accepted and that may first have to be discussed.

Epimenides is a liar, because he is a Cretan.

The premise implied here that “all Cretans are liars” would at least be worth discussing and would probably not stand up to closer scrutiny, at least in this strict form (as a universal statement).

It is not difficult to see that such “hidden” arguments lend themselves to unfair use, for example to poison a discussion with insinuations, or to imply baseless claims.

Implied premises, such as those discussed above, are usually avoided in logic in order to avoid fallacies of ambiguity. However, there are situations in which they can occur unexpectedly:

A so-called “existential import” occurs when a propositional form that does not imply existence (e.g. a universal proposition) is transferred into a form that does (e.g. into an existential proposition). If this is only done implicitly (as an enthymeme), it can lead to the existential fallacy.

An example of a syllogistic form, where existence must first be explicitly proved, is the Modus Barbari.

• Existential fallacy

• Enthymeme on Wikipedia