Problems of nature, matter and motion are typical concerns of ontology and have been widely discussed on this plane in the contemporary philosophic tradition. Phenomenologists, however, with some notable exceptions, have had little to say about ontological issues. The reason is not that they are, in general, unaware of the problems posed by this philosophic domain. On the contrary, several phenomenologists, notably Heidegger, are deeply interested precisely in this topic, which is even central to their thought. But, as the domain of ontology is not wholly separable from other regions of philosophy, other factors — in particular the form of epistemology defended — strongly influence what can be said on the ontological plane. For Aristotle, of course, ontology or metaphysics was not only a permissible but a necessary area of study. But, other forms of epistemology would seem to prohibit any explicit claims about the nature of what is.