In his Prior Analytics, Aristotle developed what has become known as his “syllogistic,” a word derived from the Greek sullogismos (deduction) or “syllogism” in English. He defines it thus: “A deduction is a discourse in which, certain things being stated, something other than what is stated follows of necessity from their being so” (Posterior Analytics I.2 24b18–20). His syllogistic is a metalogical theory in which certain argument schemas are meant to capture the form of any deductive inference. Although this is no longer believed to be a successful enterprise, Aristotle is nevertheless credited with the invention of schematic letters for predicates and subjects (“terms”), which he thinks together can represent singular and universal propositions of either positive or negative modes. Schematically, a universal affirmative proposition has the form “Every S is P,” while a singular affirmative has the form “Some S is P,” and so on. A syllogism in standard form has two premises and a conclusion in which terms are distributed in various ways. Since a universal affirmative proposition has come to be called an “A” proposition, one standard deductive schema is now called Barbara. Aristotle writes: “If A is predicated of every B, and B of every C, A must be predicated of every C.” Aristotle's logical theory is far more complex and subtle than a brief exposition can suggest, and although it is imperfect, it had a profound influence for many centuries, especially on Scholasticism.

Descartes is highly critical of syllogistic reasoning, which he associates with Scholastic logicians or what he calls “dialecticians,” a pejorative term meant to imply that they were more interested in verbal disputes and sophistry than in the truth. The main problem with formal argumentation of any kind is that it dims the light of reason, which consists in perceiving something clearly and distinctly (see clarity and distinctness). In another memorable metaphor from the Rules, Descartes says that when trafficking in argument forms, our reason “takes a holiday” (AT X 405–6, CSM I 36). Like the ancient skeptics (and anticipating Mill), Descartes also thinks that syllogisms are circular; there is a strong sense in which the conclusion is already contained in the premises, and hence such arguments do not provide a means for discovering new knowledge.