This is, in my opinion, the main flaw with Naturalism. If Naturalism is true, then no truth can be discerned (not even the concept of Naturalism itself). It's an argument for futility. It's actually a pretty dated argument, though. C.S. Lewis even spoke on the subject:

> If all that exists is Nature, the great mindless interlocking event, if our own deepest convictions are merely the by-products of an irrational process, then clearly there is not the slightest ground for supposing that our sense of fitness and our consequent faith in uniformity tell us anything about a reality external to ourselves. Our convictions are simply a fact about us-like the colour of our hair. If Naturalism is true we have no reason to trust our conviction that Nature is uniform.