a priori vs a posteriori

A priori propositions merely require looking in one's own brain for justification, whereas a posteriori propositions require reflection upon experience of the world at large.In epistemology, a priori propositions can be known* independent of (prior to) experience, whereas a posteriori propositions can be known only on the basis of experience (after experience). Thus, the distinction between a priori and a posteriori corresponds roughly to the distinction between nonempirical and empirical knowledge.

The a priori/a posteriori distinction is applied to ways of knowing, propositions, arguments, and concepts. Because of its dependence upon verification by experience, the foundation for classifying a proposition as a posteriori is more easily grasped. An a priori concept can be acquired independently of experience, which may include, but is not necessarily confined to innate concepts.

The term 'justification' signifies that the person who believes something has an epistemic reason to thinking that the belief is true. So, a priori justification for believing a given proposition involves having a reason independent of experience for regarding that the proposition as true. Such propositions include simple perceptual, numerical, and logical relations – ice water is cooler than boiling water, blue is a colour, three plus two is five, when comparing two men the taller man is not shorter than the shorter man. One is a priori justified in believing a given proposition if, on the basis of pure thought or reason, one has a reason to consider the proposition to be true. Thus, propositions for which a priori justification obtains are necessarily confined—exclusive of mathematics and logic—to trivial situations.

* Knowledge is best defined as justified true belief. Belief signifies no more than possession of a mental state, and belief alone cannot be taken to represent knowledge.

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