Like perception, introspection, memory, and testimony, a priori justification is fallible. One might be justified in believing something a priori, e.g., that every event has a cause, that is actually false. Many physicists think that some subatomic events occur at random and so have no cause. Besides being fallible, it seems that a priori justification is defeasible, a priori justification can be defeated by further evidence. Why couldn't a priori justification be defeated by empirical, not just a priori, considerations? Something like this has actually happened, Kant was a priori justified in believing that every event has a cause but, because of developments in sub-atomic physics, we are not, and that the Greeks were, all things considered, a priori justified in accepting Euclidean geometry but we are not because of developments in cosmology.
Some philosophers changed the can in the definition of a priori
knowledge to a must in order to solve some problems. Who are these
philosophers? And what would be examples of a priori knowledge using
a"must"? I won´t go further into the problems that might arise with
the notion of a prioricity here. I will say that some philosophers
somehow change the modality in this characterization from can to must.
They think that if something belongs to the realm of a priori
knowledge, it couldn´t possibly be known empirically.
Putnam (1) e Kitcher (2). Philip Kitcher thinks that if there is such a thing as a priori justification, then “a person is entitled to ignore empirical information about the type of world she inhabits. Hilary Putnam thinks that if a person is entitled to ignore empirical information, or it is always rational for her to believe something no matter what the empirical evidence is, provided she is a priori justified in believing that thing, then a priori justification is indefeasible by experience.
Putnam and Kitcher maintain that if the belief is justified a priori, then the belief is not rationally revisable in light of experiential evidence. But, they contend, the propositions traditionally alleged to be knowable a priori, such as mathematical propositions, are rationally revisable in light of experiential evidence. Therefore, knowledge of such propositions is not a priori. Consider the empirical sources that have been alleged to justify mathematical propositions empirically: counting objects, reading a textbook, consulting a mathematician, and computer results. Each of these sources is fallible in an important respect. If belief that p is justified by counting a collection of objects and arriving at a particular result, then it is possible that recounting the collection to arrive at a different result. If belief that p is justified by a textbook or mathematician or computer result, then it is possible that to encounter a different textbook or mathematician or computer result that states that not-p. In each case, the latter result is an empirically justified overriding defeater for the belief original justification. The argument, however, is problematic on two counts. First, there are grounds for denying that it captures Kant’s conception of a priori justification since his arguments in support of a priori knowledge do not address the issue of whether experience can defeat one’s justification for believing mathematical propositions. They focus exclusively on the source of such justification. Second, it settles by fiat a substantive philosophical question since it rules out the possibility that mathematical propositions are justified both a priori and by experience
But why think that a priori justification implies either that a person who has that sort of justification is entitled to ignore empirical information or that it is always rational for her to believe what she does no matter what the empirical evidence is? A priori justification must be “independent of experience,” which implies that it must be independent of empirical evidence. But there is an interpretation of that sort of independence that does not imply that the person is entitled to ignore empirical information or that her justification will remain no matter what the empirical evidence is. Suppose being justified independent of experience simply means that experiential sources do not provide the justification, that the justification is provided solely by some non-experiential source. That does not imply that the experiential evidence could not defeat that non-experiential justification. A priori justification does not imply that the justification will remain where experience is not silent. It allows that experience might defeat a priori justification. There might be three categories of justified propositions: 1- those whose justification is wholly independent of experience, 2- those whose justification does not rest on, but can be defeated by, experience, and 3- those whose justification rests, or depends on, experience. A priori justification might be applied to categories 1 and 2. It is harder to say positively what it means, but on one standard interpretation non-inferential, a priori justification is justification based solely on understanding the proposition at issue.
The notion of a priori knowledge, construed as a notion of non-empirically grounded knowledge, is not the same as a notion of epistemic certainty. Philosophers have understood ‘epistemic certainty’ in various ways: for instance, as epistemically indubitable belief or as self-evident belief. A belief is epistemically indubitable if and only if it would not be epistemically justifiable to doubt that belief under any circumstance. It is not obvious that a priori warrant for a proposition requires epistemic indubitability of this proposition. A priori justification for a proposition apparently can be subject to ‘epistemic defeat’ given a change in a priori evidence. A self-evident proposition is justified but does not depend on anything else for its justification. The problem in linking a priori warrant to such self-evidence is that a priori warrant is compatible with inferential warrant, wherein a proposition owes its warrant to inferential relations with other propositions, as might a theorem in a mathematical system.
The notion of a priori knowledge depends on a notion of a priori warrant, not on a notion of a non-empirical origin of the concepts. A notion involving special conditions for the justification of a believed proposition is not automatically a notion involving special conditions for either the origin or one’s understanding.
The primary dispute between apriorists and radical empiricists is over the source of the knowledge in question. A theory of a priori knowledge requires limitation of the set of propositions knowable a priori. Such a theory must avoid confusing the notion of what is a priori with the notions of what is necessarily true, what is analytically true, what is innate, and what is certain. It must also draw a clear distinction between what is a priori and what is a posteriori. Many prominent apriorists maintain that truth conduciveness is a necessary condition for epistemic justification.The claim that a source of beliefs is truth conducive or that it is not error conducive is a contingent empirical claim that need be supported by empirical investigation.
(1) Putnam, Hilary. “‘Two Dogmas’ Revisited.” In Realism and Reason: Philosophical Papers. Vol. 3. New York: Cambridge University Press, 1983.
(2) Kitcher, Philip. The Nature of Mathematical Knowledge. New York: Oxford University Press, 1983.