Short for argumentum a fortiori (Latin for “argument from the stronger”), a fortiori describes an argument where the conclusion rests on a previously-proven point. The conclusion in an a fortiori argument is more certain than the point it rests on. For example, if we know with a high certainty that a person is 35, then one may argue that this person is no longer in high school. The person’s age is a given that argues for itself; the fact that they are no longer in high school is of greater certainty given the person’s general age, even if the exact year is wrong (they may actually be 34 or 36, for example).

A fortiori is vital to Foucault’s conception of the 19th-century episteme and the human sciences. If humans are the basis on which all positivist knowledge can be built, then it is even more certain and assured that humans must be able to question their own knowledge at every angle to weed out what is positive knowledge from what is not. The first proposition, that humans are the basis for positivist knowledge, does not mean that this is the only kind of knowledge humans may have. This uncertainty about the contents of knowledge makes the second proposition an inevitability.