from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

  • (Logic) 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.
  • (Philos.) Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

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

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


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

[Medieval Latin ā priōrī : Latin ā, from + Latin priōrī, ablative of prior, former.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

First attested in 1710, from Latin, literally from the former, from priori ("former")


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  • a posteriori

    May 18, 2009

  • 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

  • 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

  • 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

  • What an awesome first comment!

    March 21, 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

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

    March 26, 2012