A posteriori

1. Empirical warrant

Kant (1781/1787: A2/B3) notes that ‘opposed to [a priori knowledge] is empirical knowledge, which is knowledge possible only a posteriori, that is, through experience’ (see Kant, I. §4). Empirical knowledge is a posteriori in virtue of the kind of warrant, or justification, it requires for the proposition known: a kind of warrant somehow grounded in sensory experience. The standard approach to knowledge claims that propositional knowledge requires justified true belief. The belief condition for knowledge, according to this approach, must be appropriately related to the satisfaction of the truth condition, thereby excluding true groundless conjectures from the category of knowledge. This requirement involves a justification condition for knowledge, and this condition typically receives most of the attention from contemporary accounts of empirical knowledge.

Contemporary accounts of empirical, or a posteriori justification ordinarily seek to explain what sorts of processes (vision, memory, introspection, inference and so on) can, and perhaps standardly do, yield empirical justification for beliefs. These accounts typically assume fallibilism about empirical justification: an empirically justified contingent belief can be false (see Fallibilism). These accounts also typically assume that evidence providing empirical justification for a belief need not logically entail that belief, but can be inductive or probabilistic. Although contemporary epistemologists do not share a single account of the kind of probability appropriate to empirical justification, they largely agree that empirical evidence is defeasible, that it can cease to be justifying upon one’s acquiring broader evidence. Upon approaching an apparent large pool of water on the road, for example, one might lose one’s justification for thinking that there actually is such a pool on the road. An account of empirical justification must, in any case, identify a suitable role for sensory experience in the conferring of justification. Otherwise, the distinction between a posteriori and a priori warrant and knowledge will be unclear.

Contemporary philosophers debate whether any necessarily true proposition is knowable a posteriori. Saul Kripke, for instance, has argued that some necessarily true propositions must be known a posteriori if they are to be known at all. His main examples are true identity statements involving names: for example, ‘Hesperus is Phosphorus’. Kripke holds that such statements are necessarily true, but that they cannot be known a priori (Kripke 1980). Other philosophers have challenged the view that such identity statements are necessarily true.