Whatever radical insights existentialism and phenomenology have occasioned in philosophy as well as in science, an implicit consequence of their intellectual vitality is the question they raise regarding the nexus between philosophy and science. Nowhere is this question more clearly found than in contemporary psychiatry. The recent stir in many quarters over existential psychoanalysis is only the surface disturbance of a much deeper problem, for underlying the publicity that has attended this movement is the more important, more insistent issue of the relationship between philosophical viewpoints and systems and the role of psychiatric theory in the matrix of knowledge. What is at issue, ultimately, is the very meaning of theory itself. I am interested in exploring theory in terms of a particular perspective, that of a fundamental problem for all science, the problem of causation. In a way, the choice of causation is less than necessary, for I could as well turn to the status of "fact" or "law" or "hypothesis" as a way into the difficulties I wish to engage. But if "causation" is a half-arbitrary choice, it is no less the case that it will do very well for the purposes at hand. Causation, I trust, will prove to be the threshold to the domain of theory as well as a clue to the meaning of the contribution of existential and phenomenological philosophy to science in general and to psychiatry in particular.