Ultimately Russell himself admitted that he made his greatest efforts in the field of traditional philosophy - in epistemology, the search for the ultimate grounds of our knowledge about the world. How can we be certain that what we claim to know is true? Where lies the certainty in our experience of the world? Can even the most precise knowledge - such as mathematics - be said to rest on any sure logical foundation? These were the questions that Russell sought to answer during the periods of his most profound philosophical thinking. They have remained the perennial questions of philosophy from Plato and Aristotle through Descartes, Hume, and Kant, to Russell and Wittgenstein.