Usage
In the natural sciences and in social and other human sciences where statistics plays a large role, the phrase is used to mean "even more likely" or "with even more certainty". For example, if a scientist observes that certain phenomena are present together for a certain percentage of the time, he may argue that each individual phenomenon will a fortiori be singly present for a greater or equal percentage of the time (because the latter figures, but not the former, will include the occasions on which a given phenomenon is present but one or more of the others are not).
In the art of rhetoric, i.e., speaking or writing for the acknowledged primary purpose of persuasion, the a fortiori argument draws on the speaker's and/or listener's existing confidence in a proposition to argue for a second proposition that is implicit in the first, "weaker" (less controversial and more likely to be true) than the first proposition, and therefore deserving of even more confidence than the speaker and/or listener places in the first proposition. The Christian apostle Paul makes frequent use of the argument for purpose, often signaling it with the phrase "... if, then how much more " (New International Version translation; see, e.g., 2 Corinthians 3:7–8 and 9 and Romans 5:9 and 10.)
In classical logic, truth value is binary (either absent or present, without further elaboration), as opposed to quantifiable on discrete or continuous scales as to existence and/or degree (i.e., either absent or present in some quantity that depends on the likelihood of a proposition's truth and/or the degree to which a descriptive statement applies). In classical logic, "a fortiori" is a signal indicating an attempt to justify an inferential step by claiming that the point being proven follows "from a stronger " or has been stated "by means of stronger ." That is, the phrase indicates that a) a proposition previously given or proven in the argument contains and implies a variety of "weaker" or less contentful propositions and b) the proposition being proven is only one of the propositions contained and implied.
There are two types of the a fortiori argument:
- a maiore ad minus: "from greater to smaller"
- a minore ad maius: "from smaller to greater"
Read more about this topic: Fortiori
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