Definition of a fortiori in US English:

a fortiori

adverb & adjective

  • Used to express a conclusion for which there is stronger evidence than for a previously accepted one.

    as adverb ‘they reject all absolute ideas of justice, and a fortiori the natural-law position’
    • ‘Were it not so, we should have required the a fortiori reasoning for the third degree only.’
    • ‘There is a fortiori no meaningful content to the ‘proposition’ that such a distance has changed and still more so no meaningful content to the ‘proposition’ that such a distance is changing either at a uniform or at a non-uniform rate’
    • ‘That's an argument a fortiori: If something less likely is true, then something more likely will probably be true as well.’
    • ‘What then is the conclusion which in true a fortiori fashion is supposed to follow resoundingly from the weaker premise?’
    • ‘In this way the connexive concept of implication accounts for a necessary presupposition of all conditional and a fortiori logical orientation.’


Early 17th century: Latin, from a fortiori argumento ‘from stronger argument’.