Niels Bohr, the founding father of modern atomic physics and quantum theory, was as original a philosopher as he was a physicist. This study explores several dimensions of Bohr's vision: the formulation of quantum theory and the problems associated with its interpretation, the notions of complementarity and correspondence, the debates with Einstein about objectivity and realism, and his sense of the infinite harmony of nature. The author's chief attention is given to Bohr's epistemological lesson, the conviction that all our description of nature is dependent on the words we use and the ways we can unambiguously use them. Against those who would view Bohr as a vague positivist, the author argues here that Bohr is best understood as using transcendental arguments and shaping a kind of descriptive metaphysics in his defence of our abilities to offer a description of the world we live in.