In the past century, the prevailing view has been that the main problem of epistemology, or theory of knowledge, is to give a definition of knowledge, and the prevailing definition of knowledge has been that knowledge is justified true belief. This contrasts with the fact that, since antiquity, the definition of knowledge as justified true belief has been known to be subject to counterexamples, and all the variants of this definition that have been put forward are also subject to counterexamples. As an alternative, this chapter argues that the main problem of epistemology is to explain what role knowledge plays in human life. Specifically, unlike justified true belief, knowledge in not merely a state of mind, but rather a response to the environment that is essential for survival. This involves a naturalistic approach to knowledge, according to which knowledge is a natural process, continuous with the biological processes by which life is sustained and evolved, and has a vital role, in the literal sense that life exists only insofar as there is knowledge.