Hume also follows a naturalist line in examining reason in animals. Rather than perceive reason as a special faculty of the human mind that allows us to see into the truth, Hume interprets reason as a faculty that has arisen naturally, one that helps us get by in the practical world. Animals, too, have this ability to reason, and we find it most distinctly in our inferences from causes to effects. As Hume has argued already, these inferences are not rationally justified, but are, rather, useful determinations of thought. In this section, he goes so far as to liken them with instincts. Human reason then differs from animal reason only in the sharpness and precision of our ability to infer necessary connections in nature and to think about them.
We might identify this argument with the skeptical line in Hume's thought. Contrary to rationalist philosophy, Hume argues that our reason is not a truth- tracking device that can a priori understand the many mysteries of the universe. Rather, it is simply a tool that guides us through life. All our higher reasoning is based on perceiving necessary connections in nature, and yet we never perceive any necessary connections that go beyond constant conjunction. Thus, our higher reasoning has no more rational justification than that of animals.