A priori

  • A priori is Latin for what is before. In English, we use it to describe ideas, arguments, and assumptions that are based on conjecture, prejudice, or abstract reasoning rather than real-world experience. The opposite of a priori is a posteriori, which describes ideas that are based on experience.


    A priori is a long-established loan phrase, so it’s usually not italicized. But it is italicized more often than other longstanding loanwords, probably because the a is easily mistaken for the English indefinite article.

    Meanwhile, a one-word form, apriori, has gained some ground and now appears a significant fraction of the time (about once for every 20 instances of the two-word form in all 21st-century texts indexed in Google Books). It has given rise to several derivative words, including apriority (the quality of being derived in the mind instead of from experience) and apriorism (the use of a priori reasoning).



    But when our correspondent proceeds to flying-machines, we have no longer the smallest taper-light of credible information and experience left, and must speak on a priori grounds. [Ralph Waldo Emerson, letter published in The Dial (1843)]

    In fact the a priori reasoning is so entirely satisfactory to me that if the facts won’t fit in, why so much the worse for the facts is my feeling. [Erasmus Alvey Darwin in a letter to his brother, Charles Darwin (1859)]

    All the knowledge that we can acquire a priori concerning existence seems to be hypothetical: it tells us that if one thing exists, another must exist, or, more generally, that if one proposition is true, another must be true. [Bertrand Russell, The Problems of Philosophy (1912)]

    Typically, the person putting forth the rule assumes a priori that the rule is valid, and thereafter it seems that no amount of evidence or argument can change their mind. [Arrant Pedantry (2011)]

    And can a socialist government ever adequately determine a priori the incentives a private company needs to justify the risk and effort of investing? [Financial Times (2012)]

    They involve so much complexity and ambiguity that hypothesized solutions can rarely be “proven” right a priori. [Forbes (2012)]


    1. Christian says

      I believe you might have made a mistake in the second paragraph:

      “The opposite of a priori is a posteriori, which
      describes ideas that are based on experience. So “yesterday was a boring
      day” is an a priori statement, and so is “every puppy I’ve met has
      smelled nice.” 

      I believe it should say that “yesterday was a boring day” is an a POSTERIORI statement, right?

      I love your website and always get your posts via RSS, sharing them among my English Language Department, thank you!

    2. I’m surprised that you don’t mention that the phrase has made itself so at home in the
      English language that it has turned into a word, “apriori”, (which now gets more Google hits than “a priori”), and has spawned derivatives such as “aprioricity”.

      Also, coming from a philosophical background, I was quite taken aback by the
      idea that the apriori is associated with conjecture and prejudice, and also by
      the sentence “Not all a priori knowledge is fallible, though.” In
      fact, no apriori knowledge is fallible.

      Philosophers distinguish between apriori and aposteriori knowledge, i.e.
      knowledge that doesn’t or does require examination of the world for its
      justification. They also distinguish between analytic and synthetic truths
      (i.e. truths which depend only on the meanings of the terms contained in their
      statement, and those that also require reference to the state of the world).
      And they distinguish necessary truths (which couldn’t be false) and contingent
      truths (which might have been false, but happen to be true). Philosophers spend
      quite a lot of time arguing about whether these three distinctions line up
      precisely*, or whether there can, exceptionally, be oddities like apriori
      synthetic truths. But they all agree that they generally do line up, and in particular
      that apriori knowledge is absolutely certain knowledge, whereas aposteriori
      knowledge is always, in principle, fallible. (Except, just perhaps, claims
      about what you yourself are perceiving, right now.)

      If I look at your examples and use “apriori” as I am used to doing in
      philosophical contexts, I’d have to disagree with you about almost every one.
      “Tomorrow is going to be a boring day”, “all puppies smell
      nice”, “yesterday was a boring day”, “every puppy I’ve met
      has smelt nice”, and “Abraham Lincoln never travelled to Mars”,
      are, for me, all aposteriori statements – their truth, if they are true, is
      established empirically, by direct examination of the state of the world, or by
      the best available indirect reasoning about the state of the world in some
      remote place or time.

      “2+2=4” is indeed apiori, but absolutely not “because numbers
      are intangible and because we can’t say with certainty that there are no
      exceptions in the universe.” If we couldn’t say with absolute certainty
      that there were no exceptions in the universe, it would have to be an
      aposteriori statement; it is precisely because we can say without examining the
      universe that there are no exceptions that it is apiori. (What would a
      counterexample look like? If we see four droplets of water running down a
      window-pane, and they merge into two, we don’t say “Oh dear, I was wrong
      about 2+2=4”, we say “Oh, look – water droplets don’t have a
      continuing personal identity – they can merge together and split apart.”
      And if we find a successful alien technological culture which uses the numbers
      0, 1, 2 and 3 and the sign + exactly as we do, but their textbooks say
      “2+2=10” instead of “2+2=4”, we don’t for a moment doubt
      our own arithmetic – we conclude that they write their numbers in base 4, not base
      10. (Except they, of course, insist that they do use base 10, because they call
      “10” what we call “4”.)

      If “two objects can’t occupy the same space at the same time” is
      apriori, for me it certainly isn’t because we haven’t tested it out with every
      pair of bodies in the universe. (If that test was relevant, the statement would
      have to be aposteriori.) I tend to agree that it is apriori, but only because
      of the relevant meaning of the word “body”. If two things can occupy
      the same space at the same time, such as a body of water and the body of a
      sponge, we just say that they are not both bodies in the required sense, or
      that “the same place” isn’t being used in a refined enough sense.

      It occurs to me that we could remove most of our disagreements if we could
      establish that “a priori” was reserved for your “before the event” meaning and
      “apriori” for the philosophers’ “without examining the world” meaning. It would
      help that (at least for me) they are pronounced
      differently (“ay’ prio’ry”, with the final “y” pronounced like the name of the
      letter “i”, and “apri’ory”, with the final “y” pronounced like the name of the
      letter “e”.) In fact, I suspect that I do actually use that distinction myself,
      but I have no authority for it, I don’t think it is widespread, and I have
      little hope of making it universal.



      (P.S. Great web-site! I can’t remember how I happened upon it, but I find that
      once you are in, it’s hard to leave, because it is so fascinating. I’ve learned
      a lot.)

      * Philosophers also argue about whether each of the three distinctions is
      itself an absolutely cast-iron, all-or-nothing affair, or whether there are gradations
      or trade-offs, so that for instance we can take one statement as necessary and another
      as contingent, or vice-versa.

      • Grammarist says

        (Four months later.) Thank you, Ken. We have expunged our own clumsy philosophy talk from this post for now and will expand it again when one of us has time to give these ideas more serious consideration.

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