Conclusion IX - The truth of knowledge consists in the conformity of the mind with the thing. It is absurd to doubt the reliability of our organs of knowledge (Introduction to Philosophy, 129).This reflects an objective epistemology. Our knowledge is true to the extent that it conforms to reality. Any other notion is, it seems to me, ridiculous.
Maritain has some strong words for those who doubt the "the reliability of our organs of knowledge:"
[I]t would obviously be waste of breath to attempt to demonstrate [the reliability reason and the intellect] to them. For every demonstration rests on some previously admitted certainty, and it is their very profession to admit of none. ... When they say that they do not know whether any proposition is true, either they know that this proposition at any rate is true, in which case they contradict themselves, or they do not know whether it is true, in which case they are either saying nothing whatever, or do not know what they say. The sole philosophy open to those who doubt the possibility of truth is absolute silence - even mental. That is to say, as Aristotle points out, such men must make themselves vegetables. No doubt reason often errs, especially in the highest matters, and, as Cicero said long ago, there is no nonsense in the world which has not found some philosopher to maintain it, so difficult is it to attain truth. But it is the error of cowards to mistake a difficulty for an impossibility (ibid., p. 128f.).I'm not able to go toe-to-toe with such skeptics in an actual philosophical argument, beyond resorting to one of the three ways Maritain suggests (p. 128) for dealing with them: make a reductio ad absurdum. None of them actually lives in a way consistent with what they claim. They do not doubt whether the sun has risen. They do not doubt whether they are clothed. They do not wonder whether the eggs they've cooked are actually poison. Yet if they were really consistent with their claims, they couldn't possibly eat for fear that they might be eating garbage. They couldn't breathe for fear that the air might be toxic. They couldn't love, because they might be misunderstanding the other's expressions of hatred as terms of endearment. It's all ridiculous in my book. Of course common sense isn't an argument per se, but it's unlikely for something to pass the smell test if it's as silly as the skeptics' claims.
Conclusion X - The formal object of the intellect is being. What it apprehends of its very nature is what things are independently of us (ibid., p. 133).Again: objectively valid knowledge. We're not trapped in subjectivism.
It is that which is, which causes the truth of our minds. Reason is capable of attaining with complete certainty the most sublime truths of the natural order, but with difficulty and only when duly disciplined (ibid., p. 131).It is the error of the rationalists to suppose that all truth is easily attained. The history of philosophy (and see Maritain's appeal to Cicero mentioned above) ought to disabuse us of this idea.
It seems to me that what Maritain describes in his epistemology, and earlier in his discussion of the reliability of the data we receive from the senses, is consistent with the fact of creation. I can't imagine why a Christian would object to this. God created us in such a way that we are able to live in this world. We are well suited to it, and our bodies are adapted for helping us to live "truly" in creation. This is one more indication of the fact that God loves us: he has not made us or the world in such a way that we cannot live in it.