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)]

5 thoughts on “A priori”

  1. 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.

    • (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.


Leave a Comment