A Fortiori Argument - Meaning

Meaning

When the phrase includes the Latin or English noun, it properly denotes a proof in which one demonstrates a claim by invoking as proof an already proven, stronger claim. (Example: "When one argues that if it is forbidden to ride a bicycle with an extra passenger, it is also forbidden to ride a bike with fourteen extra passengers, one makes an argument a fortiori.)

The Latin prepositional phrase taken by itself (without "argumentum"/"argument") is adverbial. (Example: "As I've personally observed, Bob can lift a 100-pound object of size, shape, and weight distribution identical to those of this object. This object weighs only 50 pounds; a fortiori, Bob can lift it also.") So used, the phrase conveys two types of meaning, one within the sentence taken at face value and one extending beyond the sentence's face value:

• First, it adds to the sentence's propositional content by modifying a modal verb such as "is" (or the modal portion of an auxiliary verb phrase such as "can/may "), contributing "'why' information" about the objective state of affairs described.

• Second, and less overtly, the prepositional phrase calls the listener's attention not merely to the explanatory information within the proposition but to the proposition itself in its capacity as an assertion – to the fact that it is being asserted and, indirectly, to the asserter's reasons for asserting it. In this capacity, it sheds light on the asserter's subjective mental state, providing information on subjects including why the asserter believes the weaker proposition, why the asserter's effort at persuasion features the stronger proposition, and why (in the asserter's actual or purported opinion) the person being persuaded should believe the weaker proposition. The difference between the logical and the personal appeal is expressible as the difference between "A and B; a fortiori, A" on one hand and "A and B; a fortiori, 'A'" (note "scare quotes") on the other. More fully, the personal appeal runs "I believe that ; I am led 'by th stronger ' and relevant rules of inference to believe that, and I hereby invoke it as justification for the assertion, which I hereby make, that . I understand you to likewise believe that and to endorse these same rules of inference. Therefore, by your own logic, you have even more reason to believe that than you do to believe that, and you should need no external persuasion to believe that ."