According to Michael Rea, naturalism is a combination of a method and an assumption. Methodologically, naturalism is the attempt to understand the world through the natural sciences. The assumption, which, theoretically at least, can be overturned at any moment, is that everything that exists is material. The assumption can, in principle, be overturned at any moment because natural sciences don't claim to have all data at their fingertips. Fairies could be found at the bottom of an English garden and God might appear from behind a far off planet, but neither of these events seems plausible given only the data that the natural sciences have produced so far. Rea defines naturalism as a research program. As such it does not fit the bill as a substantive thesis. To say that naturalism is a substantive thesis would be to suggest that it is a thesis about metaphysics, epistemology or methodology. Rea argues that any such thesis is incompatible with naturalism. Consequently, naturali...