Naturalism assumes that there is no higher order beyond empirical nature, and, as a result, no basis for natural teleology (viz., an order in nature which points toward a higher intelligible order). It follows that we qua natural subjects cannot learn anything from nature, since, literally, naturalism forbids observing from empirical nature to anything higher. Nature, in other words, is not a teacher.
Yet, man is a learner. If nature teaches nothing, how can man learn anything from it? Expressing empirical observations is not learning, since learning involves grasping principles, whereas observation merely involves sensory impressions. Only by positing a higher intelligible continuity that transcends any and all instances of empirical observation can those observations be constructed into a theory (i.e., a syllogism of learned principles). It follows that no series or set of observations can be constructed into a theoretical defense of naturalism itself.
So, either humans are not simply natural, or humans have no way of learning the truth of naturalism from nature.
A little Lewis, a little Plantinga, a little Reppert.