Before we can consider whether the senses are reliable, we first have to have an idea of what truth is. St. Thomas says that truth consists in the conformity of the intellect to the thing known (ST I Q16 A1). What this means is that our thoughts about a thing are only true if they conform to how that thing really is. Truth, therefore, is anchored in the world as it really is. To say something like "Well, that may be true for you, but it's not true for me" is to say something irrational. Truth is not subjective.
It might be worth clarifying that this is not precisely how things stand with respect to God's knowledge: it's backwards.
Now, in things, neither truth nor falsity exists, except in relation to the intellect. And since every thing is denominated simply by what belongs to it "per se," but is denominated relatively by what belongs to it accidentally; a thing indeed may be called false simply when compared with the intellect on which it depends, and to which it is compared "per se" but may be called false relatively as directed to another intellect, to which it is compared accidentally. Now natural things depend on the divine intellect, as artificial things on the human. Wherefore artificial things are said to be false simply and in themselves, in so far as they fall short of the form of the art; whence a craftsman is said to produce a false work, if it falls short of the proper operation of his art.All things conform with the divine intellect, and in that respect they are true (excepting the sinful acts of voluntary agents, which are false). With relation to us, though: since truth is found in the intellect (in its conformity to the thing), then falsity also must be found there.
In things that depend on God, falseness cannot be found, in so far as they are compared with the divine intellect; since whatever takes place in things proceeds from the ordinance of that intellect, unless perhaps in the case of voluntary agents only, who have it in their power to withdraw themselves from what is so ordained; wherein consists the evil of sin. Thus sins themselves are called untruths and lies in the Scriptures, according to the words of the text, "Why do you love vanity, and seek after lying?" (Psalm 4:3): as on the other hand virtuous deeds are called the "truth of life" as being obedient to the order of the divine intellect. Thus it is said, "He that doth truth, cometh to the light" (John 3:21) (ST I 17 A1; emphasis added).
Since true and false are opposed, and since opposites stand in relation to the same thing, we must needs seek falsity, where primarily we find truth; that is to say, in the intellect (ibid).Well, do the senses "know truth"? St. Thomas says that they do not.
As stated before, truth resides, in its primary aspect, in the intellect. Now since everything is true according as it has the form proper to its nature, the intellect, in so far as it is knowing, must be true, so far as it has the likeness of the thing known, this being its form, as knowing. For this reason truth is defined by the conformity of intellect and thing; and hence to know this conformity is to know truth. But in no way can sense know this. For although sight has the likeness of a visible thing, yet it does not know the comparison which exists between the thing seen and that which itself apprehends concerning it. But the intellect can know its own conformity with the intelligible thing; yet it does not apprehend it by knowing of a thing "what a thing is." When, however, it judges that a thing corresponds to the form which it apprehends about that thing, then first it knows and expresses truth. This it does by composing and dividing: for in every proposition it either applies to, or removes from the thing signified by the subject, some form signified by the predicate: and this clearly shows that the sense is true of any thing, as is also the intellect, when it knows "what a thing is"; but it does not thereby know or affirm truth. This is in like manner the case with complex or non-complex words. Truth therefore may be in the senses, or in the intellect knowing "what a thing is," as in anything that is true; yet not as the thing known in the knower, which is implied by the word "truth"; for the perfection of the intellect is truth as known. Therefore, properly speaking, truth resides in the intellect composing and dividing; and not in the senses; nor in the intellect knowing "what a thing is" (I Q16 A2).But we could say that the senses "know truth" in another sense - that is, in whether they apprehend sensible things truly.
Now truth is not in them in such a way as that the senses know truth, but in so far as they apprehend sensible things truly, as said above (16, 2), and this takes place through the senses apprehending things as they are, and hence it happens that falsity exists in the senses through their apprehending or judging things to be otherwise than they really are (I Q17 A2).But this is something rare or unusual, he says, or a consequence of our attention being directed elsewhere (for example).
The upshot is that our senses are in fact reliable. And we all take this reliability for granted: we trust them all the time. We do not falter upon approaching a green light, suspicious whether it might actually be red (unless we already know that, being color blind, our sight is defective in this respect). We know that it is green. Our senses reliably report the world to us. Even those who dispute this in theory do not really question it when it comes to really living, and this ought to tell us something about how seriously we should take their theories.
And really, it ought to ... make sense ... to us that our eyes and ears are trustworthy for telling us about creation. Because God made the world, and made us to live in the world. He is all good, and he loves us, and having made us to live in this world, we are actually equipped to do so.
The problem when it comes to our senses isn't our senses themselves but rather what we do with what they report to us. We misinterpret things. We draw conclusions on insufficient evidence. We reason badly in other ways. Making right use of reason is hard work, and in our intellectual laziness we are often careless about understanding the world around us. Sometimes we don't even try to reason about such things, and instead try to wedge them into some preconceived notions we have about how we think the world is. In consequence we are found to be in error - but these errors cannot be blamed upon our senses.