Philosophy of St. Thomas - Knowledge of the Physical World

I'm not sure that I can do this topic justice, owing to the inadequacy of my education in philosophy. However, it's important enough that I want to try.

St. Thomas insists that we do have real knowledge of the physical world.

Some have asserted that our intellectual faculties know only the impression made on them; as, for example, that sense is cognizant only of the impression made on its own organ. According to this theory, the intellect understands only its own impression, namely, the intelligible species which it has received, so that this species is what is understood.

This is, however, manifestly false for two reasons.

First, because the things we understand are the objects of science; therefore if what we understand is merely the intelligible species in the soul, it would follow that every science would not be concerned with objects outside the soul, but only with the intelligible species within the soul; thus, according to the teaching of the Platonists all science is about ideas, which they held to be actually understood.

Secondly, it is untrue, because it would lead to the opinion of the ancients who maintained that "whatever seems, is true" [Aristotle, Metaph. iii. 5], and that consequently contradictories are true simultaneously. For if the faculty knows its own impression only, it can judge of that only. Now a thing seems according to the impression made on the cognitive faculty. Consequently the cognitive faculty will always judge of its own impression as such; and so every judgment will be true: for instance, if taste perceived only its own impression, when anyone with a healthy taste perceives that honey is sweet, he would judge truly; and if anyone with a corrupt taste perceives that honey is bitter, this would be equally true; for each would judge according to the impression on his taste. Thus every opinion would be equally true; in fact, every sort of apprehension (ST I Q85 A2).

It's not too hard to see here an obvious rejection of what eventually became the stumbling block of modernity: that is, skepticism. Contrary to this, St. Thomas says that we do know the real world:

Therefore it must be said that the intelligible species is related to the intellect as that by which it understands: which is proved thus. There is a twofold action (Metaph. ix, Did. viii, 8), one which remains in the agent; for instance, to see and to understand; and another which passes into an external object; for instance, to heat and to cut; and each of these actions proceeds in virtue of some form. And as the form from which proceeds an act tending to something external is the likeness of the object of the action, as heat in the heater is a likeness of the thing heated; so the form from which proceeds an action remaining in the agent is the likeness of the object. Hence that by which the sight sees is the likeness of the visible thing; and the likeness of the thing understood, that is, the intelligible species, is the form by which the intellect understands. But since the intellect reflects upon itself, by such reflection it understands both its own act of intelligence, and the species by which it understands. Thus the intelligible species is that which is understood secondarily; but that which is primarily understood is the object, of which the species is the likeness. This also appears from the opinion of the ancient philosophers, who said that "like is known by like." For they said that the soul knows the earth outside itself, by the earth within itself; and so of the rest. If, therefore, we take the species of the earth instead of the earth, according to Aristotle (De Anima iii, 8), who says "that a stone is not in the soul, but only the likeness of the stone"; it follows that the soul knows external things by means of its intelligible species (ibid.)

Something similar is said by Pope Benedict in Jesus of Nazareth:

Knowing always involves some sort of equality. "If the eye were not sunlike, it could never see the sun," as Goethe said, alluding to an idea of Plotinus. Every process of coming to know something includes in one form or another a process of assimilation, a sort of inner unification of the knower with the known (p. 340).

To try and put these two together: the equality of which the Pope speaks is - for purposes of our knowledge of our world - accomplished through the mediation of the senses, by which we form "phantasms" (mental images or representations of real objects) and arrive at intelligible species - by means of which we know external things.

What is an "intelligible species"?

Intelligible species are the intellect's versions of the forms which give those extramental things their nature or quiddity (source, towards the bottom of the page).

Well this whole post seems pretty convoluted to me, unfortunately, so I have to apologize for the lack of clarity. The point, however, is that by means of our senses we achieve real knowledge of the world. I have written about this more than once, but I do so because it's important. There has to be a correspondence between the world and what we know about the world, or we would be unable to live in it. Furthermore, as Aquinas points out, it's fundamentally irrational to suggest otherwise: it means that two people could say contradictory things about the world ("honey is sweet"/"honey is bitter") and both could be "true". But this is impossible.