Oedipus notoriously brought on his own tragic end because of the driving force of his will to know his own identity, a grim illustration of the comment with which Aristotle opens his Metaphysics: “all humans, because of their very nature, yearn for knowledge”. The word Aristotle uses here for knowing is eidenai, which has a root connection with the verb for “seeing”: a knowing that, grasped by the rational mind. This contrasts with nouns epistēmē, a knowing how, connected with scientific understanding, and also gnōmē or gnōsis, recognition from acquaintance, noēsis, intellectual activity, and phronēsis, practical wisdom. These terms oft en overlap, and, in their multiplicity, we find that the Greeks continually raised questions about knowledge and the different kinds of knowing. They explored the contrast with doubt and opinion, the part played by perception, especially sight, in guaranteeing the validity of knowledge, the relationship of the subject (a “knowing mind”) to the object (“what is known”), how knowledge should be defined, and the possibility of “knowledge of knowledge”, a master science that would bring with it a “theory of everything”. The recognition of such problems regarding the basis and validity of knowledge and the attempts to answer them, which relate to the branch of philosophy known as epistemology, were of central importance to the main Greek philosophers, and count among their greatest achievements.