"The issue of realism is at the heart of metaphysics; that of rationality is at the heart of epistemology. Neither of these issues can be isolated from the other, nor can we separate epistemology and metaphysics. Our account of what there is constrains our theory of rational belief, and hence of rationality in general; and our theory of rational belief constrains our ontological outlook. It may be, however, that philosophers naturally tend to take one or the other of these two philosophical domains, epistemology or metaphysics, or some account developed therein, as primary. If we give priority to epistemology, we tend to produce an ontology that posits the sorts of objects about which our epistemology says we can have knowledge or justified belief; and if we give metaphysics priority, we tend to produce an account of rational belief which allows knowledge or justified belief about the sorts of things our ontology countenances as real."
Robert Audi, "Realism, Rationality, and Philosophical Method," Proceedings and Addresses of the American Philosophical Association, Vol 61 No 1 (September 1987): 65-74.