The book develops a conception of epistemology in which the notion of knowledge is explanatorily fundamental. It reverses the traditional programme of trying to analyse knowledge as a combination of truth, belief, and other factors, such as justification. Rather, belief is a state whose successful form is knowledge, and justification is on the basis of knowledge, which is acquainted with evidence. Knowing is as much a mental state as believing, but it is world-involving because one can know only what is true; the book extends the externalist conception of mind from the contents of mental states to the attitudes to those contents. As with other mental states, one cannot always know whether one is in the state of knowing. It is argued that this is a special case of a much more general phenomenon; no non-trivial conditions are such that one is always in a position to know that they obtain whenever they in fact do so. This result has disturbing implications for the nature of rationality, because one is not always in a position to know what it is rational to do. Traditional arguments for scepticism fail because they assume that one is always in a position to know what one's evidence is. The speech act of assertion is also governed by a norm of knowledge. A final chapter explores the limits on what can be known that are revealed by the so-called paradox of knowability.