This final section gives us a particularly clear understanding of Hume's relationship with naturalism and skepticism. While Hume concedes that certain irrefutable skeptical doubts can cast our reasoning into grave danger, our natural instincts should ultimately bail us out.
Before examining the consequent skepticism that has run throughout the Enquiry, we should briefly consider antecedent, or Cartesian, skepticism. Descartes opens the ##Meditations## by calling into doubt all the foundations of our judgments, in particular the testimony of the senses. The end of the First Meditation leaves us wondering if there is anything certain at all. In the Second Meditation, Descartes' Meditator assures us that we cannot doubt our own existence, and from this bedrock of certainty deduces the existence of God, of the external world, and all of Cartesian metaphysics.
Hume criticizes this approach, suggesting first that there is no secure first principle that is beyond doubt, and second that even if there were we could not proceed beyond it. Existence and non-existence, Hume asserts, can be confirmed only in experience, not through reason alone. Reason can establish mathematical truths, but nothing more substantial, and so the claim, "I exist," requires empirical evidence. Hume goes on to suggest that even if Descartes' Meditator could prove her own existence by means of pure reason, nothing else could be shown to follow from this claim. Her powers of deductive reasoning have been called into doubt, and so cannot be relied upon to deduce further truths.
Throughout the Enquiry, Hume has been employing a kind of consequent skepticism, as opposed to Descartes' antecedent skepticism. Descartes' skepticism is called "antecedent" because it demands some firm starting point before any reasoning can begin. Hume's doubts, on the other hand, arise in the course of his investigations into human understanding. Hume asks on what grounds we base our judgments and investigates their rational justification. Finding certain holes in our normal procedures--for instance, that our belief in necessary connection is not rationally justified--Hume is led to a kind of consequent doubt of our mental faculties.
Hume takes this doubt a step further in arguing that our belief in an external world is not rationally justified. All I know of the external world is what my senses report to me, but these reports can often be mistaken. Besides, they are only mental representations of external objects, and not the objects themselves, and I have no rational justification for inferring the existence of external objects based on mental representations. Thus, Hume concludes, we have insufficient evidence for the existence of an external world.
The extreme form of consequent skepticism concludes unhappily that none of our judgments are rationally justified. The only sensible thing to do in that case would be to suspend all judgment and to stop acting altogether. If I have no reason to think one thing rather than another or to do one thing rather than another, I am rendered completely immobile.