Hume distinguishes between two kinds of skepticism: antecedent and consequent skepticism, both of which come in an extreme and a moderate form. He identifies the extreme form of skepticism with the universal doubt of ##Descartes##, which calls into question all former opinions and even the testimony of the senses. No claim is acceptable to the Cartesian skeptic unless is can be deduced from some indubitable first principle. Hume suggests that, first, there is no first principle that is so self-evident as to be beyond doubt, and second, even if there were such a first principle, we couldn't advance beyond it, having not yet rescued from doubt our ability to reason deductively.
Though this extreme antecedent skepticism is unworkable, Hume commends it in a more moderate form. It consists simply in forming unprejudiced opinions, progressing by small steps from sound first principles, and examining one's conclusions frequently and carefully.
The skepticism of the Enquiry has been instead a kind of consequent skepticism, that questions our habitual conclusions and judgments by doubting the grounds on which they are secured. Hume considers in particular the testimony of the senses, which suggests to us the existence of a world external to and independent of our senses. We are led by a powerful instinct to suppose that what our senses report to us is an accurate representation of this external world. However, not only do our perceptions change as we move about in the world, but there are cases of dreams or madness where our senses deceive us entirely. We can only justify our belief in an external world through experience, but experience cannot take us beyond the very perceptions that we are calling into doubt. Thus, Hume concludes, our belief in an external world is not rationally justified.
In its extreme form, consequent skepticism can lead us to complete inaction. While philosophers tend to draw a distinction between secondary qualities, such as color, sound, or texture, and primary qualities, such as extension and solidity, our understanding of both is dependent upon experience: we cannot conceive of an extended body that has no color or shape. If we doubt the testimony of our senses, we have no understanding of matter. Similarly, mathematical reasoning can lead us to counter-intuitive conclusions about space and time, presenting them to us as infinitely divisible. Consequent skepticism also leads us to doubt causal reasoning, since no conclusions that outstrip the observation of constant conjunction are rationally justified.
Such skepticism, however, dries up when we ask to what use we can put it. We cannot help but reason causally, and to refuse to do so on the basis of skeptical reasoning would lead us to desist from acting or judging altogether. Our natural instincts cannot help but reinstate what skeptical reasoning tries to dismantle.
While this extreme form of consequent skepticism is clearly unlivable, Hume again finds it useful in a more moderate form. Dogmatic and hasty reasoning may be mitigated by a constant recognition that reasoning can go astray and judgments should never be absolute. Reasoning about relations of ideas can only teach us mathematical truths, and cannot lead us to more general metaphysical principles. Reasoning about matters of fact is supported only by experience, and so we cannot provide logical proofs of the existence or non-existence of any entity. The closing line of the Enquiry urges us to ask of any book: "Does it contain any abstract reasoning concerning quantity of number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: For it can contain nothing but sophistry and illusion."
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.
Hume's naturalism rescues him from this extreme skepticism. While neither our belief in an external world nor our belief in necessary connection are rationally justified, custom and habit lead us instinctually to accept them. Skepticism is useful in that it places limitations on our reason and makes us doubt what we might otherwise take for granted, but it is ultimately unlivable. I can doubt all I please in the comfort of my study, but in order to get by in the world I must as least assume that there is an external world and that my judgments and actions in that world make some sort of difference.
Naturalism makes skepticism livable by reinstating certain kinds of thinking and reasoning as acceptable and trustworthy. Importantly, though, naturalism only reinstates relations of ideas and matters of fact, leaving metaphysics a little empty. Relations of ideas deal only with mathematical truths and matters of fact must be grounded in experience. Thus, a great deal of the subject matter of rationalist metaphysics--the existence of God, the immortality of the soul, the nature of matter, etc.--is discarded. We cannot answer such questions through reason alone, as a rationalist would want, and there is nothing in experience that can point us fruitfully toward any satisfying answers. Thus, in the closing line of the Enquiry, Hume recommends that we commit to flames all books that engage in such empty, metaphysical speculations.