A quote from The Modern Scholar: The Philosophy of Thomas Aquinas by Prof. Peter Kreeft
“Aquinas is a realist, not an idealist. He believes that reason reaches objective reality, not just our own ideas. And this epistemological realism distinguishes him from all three of the major schools of modern epistemology, all of which, in three different ways, are idealist rather than realist. First of all, Aquinas does not begin by assuming nothing and trying to prove something by deducing something about objective reality from our ideas, as Descartes’s rationalism does. Second, he does not begin by defining an idea as the first object of our knowledge, as Locke’s Empiricism does. And third, he does not agree with what Kant calls the ‘Copernican Revolution in philosophy,’ which claims that in all our thinking the thinking subject forms and structures and determines the known object rather than vice versa. Aquinas is a realist because he believes that from the beginning reason is open to reality and reality is open to reason. But he does not try to prove this, he assumes it, and he would argue that we have to assume that from the first moment we claim to know any truth at all.”