After our article "Reason as Persuasion" appeared, we received requests for more on logic, for the patterns of reasoning used to organize and present material persuasively. They belong to the rhetorical strategy logos so appeal to reason rather than to emotion (pathos) or to the speaker's credibility (ethos).

These patterns occur in informal interactions as well as in presentations, professional discussions, and documents. In both formal and informal use, they represent ways of thinking as well as ways of sharing and organizing information.


Among the patterns is deduction, a form of logic that employs the syllogism to move from the general to the specific. A syllogism makes a statement about a broad class, fits an individual or specific category into that class, and then applies the original statement to the individual or category.

This classic example of a syllogism concerns mortality and Socrates:

All men are mortal. (makes statement about a broad class: the major premise)

Socrates is a man. (places individual into the class: the minor premise)

Therefore, Socrates is mortal. (applies the statement to the individual: the conclusion)


We sometimes omit part of a syllogism, creating an enthymeme, most usually when the part omitted seems so obvious that it doesn't need to be stated. The syllogism about Socrates, for example, could be shortened to the enthymeme "Socrates is mortal because he's a man" or "All men are mortal, and Socrates is a man."

Enthymemes abound in daily conversation. "I'm in the mood for pasta," one colleague announces as noon approaches. "Let's go to Tres Amici," another replies. "Does it serve pasta?" the first asks. "It's Italian," the second answers. The omitted part of this syllogism is, of course, "Italian restaurants serve pasta."

But enthymemes also abound in formal communication situations. In both, they can sometimes create misunderstanding. Listeners may not follow your logic if one part of the syllogism is missing. Or it may be that each premise needs to be supported with evidence before an audience can accept it as sound. In those cases, pattern your argument as a syllogism.


Syllogistic reasoning can appear throughout a document or presentation in ways small and large. Speaking about a new drug, for example, one could say that all statins are metabolized in the liver, the new drug is a statin, so it, too, is metabolized in the liver. Such a syllogism could be a small part of a larger message or the major message itself.

Used formally, a syllogism requires that each premise be clearly stated and that evidence of its soundness be provided. In the statin example, such proof could consist of study results or references to accepted science. In other instances, expert testimony, examples, history, or validated research could support the premises. But evidence by which a given audience can evaluate the accuracy of the premises is needed so that the conclusion, too, is accepted as accurate.

Integral to constructing and supporting a syllogism is establishing the assumptions, foundations, and knowledge base of your audience as well as your own. Doing so allows you to stake out the common ground on which you both stand and suggests ways of widening it.

Skillful use of deductive reasoning is one way that logos can help you persuade. When your claim can be strengthened and its conclusion proven by moving from the general to the specific, take aim at your audience's intellect. As a strategy, it can indeed be brilliant.