From Edward Feser’s overview of Thomistic philosophy:
Ancient and medieval philosophy in general, and Thomism in particular, emphasized metaphysics over epistemology, and objective reality over our subjective awareness of it. The right order of inquiry, from this point of view, is first to determine the nature of the world and the place of human beings within it, and then on that basis to investigate how human beings come to acquire knowledge of the world. Modern philosophy, beginning with Rene Descartes (1596-1650), reverses this approach, tending as it does to start with questions about how we can come to have knowledge of the world and only then going on to consider what the world must be like, based on an account of our knowledge of it. In particular, both Descartes’ rationalism and the empiricism of writers like Locke, Berkeley, and Hume begin with the individual conscious subject or self, develop a theory about how that self can know anything, and then determine what reality in general must be like in line with their respective theories of knowledge.
One result of this subjectivist method was to make objective reality and common sense problematic in a way they had not been for Aristotle and Aquinas; skepticism thus came to seem a serious threat, and idealism (the view that the material world is an illusion and that mind alone is real) came to seem a serious option. Another consequence was that even when some sort of objective reality was acknowledged, doubts were raised about the possibility of knowing much about it beyond what the senses could tell us directly. Accordingly, grand metaphysical systems of the sort presented by Plato, Aristotle, and Aquinas were called into question. The philosophy of Immanuel Kant (1724-1804) was an especially influential expression of hostility to traditional metaphysics, distinguishing as it does between “phenomena” (the world as it appears to us, of which we can have knowledge) and “noumena” (the world as it exists in itself, which we cannot know).