The nature of knowledge has been a central problem in philosophy from the earliest times. One of Plato's most brilliant dialogues, the Theaetetus, is an attempt to arrive at a satisfactory definition of the concept, and Plato's dualistic ontology—a real world of eternal Forms contrasted with a less real world of changing sensible particulars—rests on epistemological foundations.

The problem of knowledge occupies an important place in most major philosophical systems. If philosophy is conceived as an ontological undertaking, as an endeavor to describe the ultimate nature of reality or to say what there really is, it requires a preliminary investigation of the scope and validity of knowledge. Only that can reasonably be said to exist which can be known to exist. If, on the other hand, philosophy is conceived as a critical inquiry, as a second-order discipline concerned with the claims of...