How can human beings, who are liable to error, possess knowledge? The skeptic finds this question impossible to answer. If we can err, then it seems the grounds on which we believe do not rule out that we are wrong. Most contemporary epistemologists agree with the skeptic that we can never believe on grounds that exclude error. Sources of Knowledge moves beyond this predicament by demonstrating that some major problems of contemporary philosophy have their roots in the lack of a metaphysical category that is fundamental to our self-understanding: the category of a rational capacity...