 As one has the intuition of being and pursues this investigation into being, one is led into the traditional questions of metaphysics and natural theology. There are, Maritain holds, four basic principles of metaphysics: the principles of identity, of sufficient reason, of efficient causality, and of finality (i.e., every agent acts for an end). Though the truth or the applicability of these principles does not admit of direct proof, they are consistently confirmed by experience and, Maritain holds, cannot be denied without contradiction (Preface to Metaphysics, 1934 [1939: 90]).