Argumentum a fortiori (pron. /ˈɑːfɔːrtɪˈoʊriː/;Latin: "from a/the stronger [thing]") is a form of argumentation which draws upon existing confidence in a proposition to argue in favor of a second proposition that is held to be implicit in the first. The second proposition may be considered "weaker," and therefore the arguer adduces a "stronger" proposition to support it.
This section requires expansion with: More examples would better illustrate this concept. (May 2016)
If a person is dead (the stronger reason), then one can with equal or greater certainty argue that the person is not breathing. "Being dead" trumps other arguments that might be made to show that the person is not breathing, such as, for instance, not seeing any sign of breathing (provided, of course, that the determination of "being dead" can be made without using "not breathing" as evidence, else this scenario becomes circularly reasoned).
If driving 10 mph over the speed limit is punishable by a fine of $50, it can be inferred a fortiori that driving 20 mph over the speed limit is also punishable by a fine of at least $50.
If a teacher refuses to add 5 points to a student's grade, on the grounds that the student does not deserve an additional 5 points, it can be inferred a fortiori that the teacher will also refuse to raise the student's grade by 10 points.
Bryan A. Garner, an authority on usage, has written in Garner's Modern American Usage that writers sometimes use a fortiori as an adjective, which he says is "a usage to be resisted." As an example of this he gives the sentence, "Clearly, if laws depend so heavily on public acquiescence, the case of conventions is an a fortiori [read even more compelling] one."