from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Proceeding from a known or assumed cause to a necessarily related effect; deductive.
  • adj. Derived by or designating the process of reasoning without reference to particular facts or experience.
  • adj. Knowable without appeal to particular experience.
  • adj. Made before or without examination; not supported by factual study.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Known ahead of time.
  • adj. Based on hypothesis rather than experiment.
  • adj. Self-evident, intuitively obvious
  • adj. Presumed without analysis
  • adv. In a way based on theoretical deduction rather than empirical observation

from the GNU version of the Collaborative International Dictionary of English

  • Characterizing that kind of reasoning which deduces consequences from definitions formed, or principles assumed, or which infers effects from causes previously known; deductive or deductively. The reverse of a posteriori.
  • Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible.

from The Century Dictionary and Cyclopedia

  • n. From the former; from that which precedes; hence, from antecedent to consequent, from condition to conditioned, or from cause to effect.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adj. based on hypothesis or theory rather than experiment
  • adj. involving deductive reasoning from a general principle to a necessary effect; not supported by fact
  • adv. derived by logic, without observed facts


Medieval Latin ā priōrī : Latin ā, from + Latin priōrī, ablative of prior, former.
(American Heritage® Dictionary of the English Language, Fourth Edition)
First attested in 1710, from Latin, literally from the former, from priori ("former") (Wiktionary)


  • Yet this rational group-formation can be disrupted under certain circumstances, since it is not of an a priori character.

    Conflict and The Web of Group-Affiliations

  • Then, as now, the so-called a priori arguments against Theism; and, given a Deity, against the possibility of creative acts, appeared to me to be devoid of reasonable foundation.

    The Life and Letters of Thomas Henry Huxley

  • It seems impossible to deny an a priori fighting instinct, especially if one keeps in mind the incredibly picayunish, even silly, occasions of the most serious conflicts.

    Conflict and The Web of Group-Affiliations

  • He would then test it by experiment, and when it failed would at once recognise that his hypothesis was a priori bound to fail.


  • This person introduced, or at all events propagated, what was termed the deductive or a priori mode of investigation.

    Mellonta Tauta

  • A poet, a scholar, a traveller, a diplomat, a famous wit, an active member of Parliament from the Restoration to his death in 1678, the life of Andrew Marvell might a priori be supposed to be one easy to write, at all events after the fashion in which men's lives get written.

    Andrew Marvell

  • The similitude of the stile and manner of it, with those my father constantly had heard preached in his parish-church, was the ground of his conjecture, — proving it as strongly, as an argument a priori could prove such a thing to a philosophic mind,

    The Life and Opinions of Tristram Shandy, Gentleman

  • Kant also provided the same kind of foundation for the metaphysics of morals as for the metaphysics of nature, for the supreme princi - ple of morality is an a priori synthetic practical propo - sition, and its possibility like that of the a priori syn - thetic propositions of the other sciences requires a demonstration in order “to prove that morality is no mere phantom of the brain.”

    Dictionary of the History of Ideas

  • As to whether it “certainly should not” mean this, does Mr. Slatin mean an a priori certainty deducible from the deep structure of the language itself or an observational certainty based on universally accepted, peer-reviewed, reproducible results obtained under controlled conditions?

    The Right Word in the Right Place at the Right Time

  • It is probably true that emotional tension often plays a larger part among persons who love a priori reasoning -- the "tender-minded" of Dr. James -- than it does in those who work through observation; but on the other hand exclusively empirical attitude has its limitations and its dangers.

    The Journal of Abnormal Psychology


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  • Haha! Okay, now I'm listing this on my cicero list.

    March 26, 2012

  • Most third declension adjectives (as distinct from nouns) have ablative signulars ending in -i. An important exception in classical Latin is the comparative adjective, which has abl. sing. in -e. But terms like a priori, a posteriori, and a fortiori may come from late Latin or medieval Latin, where that exception had been "normalized." Languages do change over time, and Cicero, great as he was, is not the eternal archetype of the language.

    March 26, 2012

  • What an awesome first comment!

    March 21, 2012

  • Thanks--I was just going to tag this why I adore this site, but now I think I'll make a new list.

    March 21, 2012

  • Thomas was not, I think, confusing the dative with the ablative. He was thinking that prior was declined like omnis, which is nonstandard but defensible as an alternative.

    March 21, 2012

  • In good Latin this was a priore. According to ΣΩΦΡΟΣΥΝΗ, who seems a reliable sort of chap, it was Thomas Aquinas who first used, or at least popularized, the dative for the ablative. It's surprising how it caught on and how rarely the Classical forms are used.

    June 3, 2009

  • a posteriori

    May 18, 2009