A FORTIORI (understood in the sense of ratione). — For a stronger reason. One of the forms of argument by analogy. It occurs when, from a certain affirmation x, one concludes to an affirmation y, because the reason that holds for x holds or seems to hold even more strongly for y. For example: Tizio is very generous in works of charity; therefore, a fortiori, he will be honest in his contracts. The a fortiori argument, while it may yield a certain conclusion, remains in its logical structure problematic, being essentially based on a relationship of similarity, which alone is not sufficient guarantee of certainty.
BIBL.: U. Viglino, Logica et generalis introductio in philosophiam, Rome 1941, pp. 302-303. Ugo Viglino