Modern philosophy, if it has not settled any other of the chronic disputes that have troubled the history of the subject, appears to have decided once and for all the question of synthetic a priori principles. Logical analysis has demonstrated that synthetic propositions are empirical while a priori propositions are analytical and notational. Nevertheless, a broader survey of the contemporary philosophical scene reveals that the strict meaning of the expression “modern philosophy” above should be rendered “philosophers of one of the current schools of philosophy”. For contemporary European philosophers have not abandoned the notion of synthetic a priori principles altogether. They have modified without abandoning Kant's Copernican discovery of the laws of nature in the human mind. There are, to be sure, two ways of viewing the situation. Either logical analysis has overlooked certain unique phenomena and thus has failed to comprehend the arguments which take their description as premises, or existentialism has persisted in the use of an inadequate logic. The purpose of this paper is to test this issue and in doing so to explore the psychological roots of the idea of synthetic a priori principles. The means adopted is a critical study of the existentialist theory of emotion which claims to have discovered a previously unrecognized basis for synthetic a priori principles in the phenomenelogy of human existence.