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In this final section of the book, Hume discusses skepticism—what it is and what its benefits and drawbacks are. Hume explains that skepticism, in general, is doubt. He divides skepticism into two types: universal doubt and consequential doubt. Each is either extreme or moderate. The main difference between the two is that with the former doubt begins prior, or antecedent to, any investigation, while the latter doubts conclusions consequent to an investigation.
Descartes's method exemplifies universal skepticism. He doubted everything, including the very faculties used in his investigation, in order to arrive at certainty. It is this inconsistency—enlisting the very faculties under suspicion to reveal their nature and veracity—that makes his approach untenable. Not only that, but, as Hume has pointed out before, reasoning cannot produce belief, which is what Descartes wants. Finally, Descartes's demand is too stringent, which produces the sort of doubt enlisted in the Meditations on First Philosophy. Taken seriously, there is no escape from it. There can be no principle immune from doubt, not even the supposed certainty of one's existence as a thinking thing. It is, then, a self-defeating sort of skepticism.
Hume does, however, propose a more moderate version of this sort of doubt, which he takes to be "a necessary preparative to the study of philosophy," dedicated as it is to "weaning our mind" of various prejudices acquired since childhood. Progress is made cautiously, with frequent checks on one's conclusions as one proceeds from solid first principles.
The second sort of skepticism is consequential doubt. Hume associates himself with this type of skepticism, discussing its results in terms of a conclusion most people take for granted: the existence of an external world. It is not unusual to follow one's instinct that there is a world independent of what is sensed and that what is sensed is an accurate representation of it. Nevertheless, such a view does not hold up to skeptical scrutiny. After all, accessing such a world would require a mode other than sense impressions, since the question is whether or not those impressions really do connect to an independent existence. One is not, Hume concludes, rationally justified in asserting a world independent of impressions. This does not mean, however, that one should really doubt that such a world exists.
Hume focuses his attention in Part 2 on the skepticism of reason. He looks to problems first introduced by ancient thinkers to show that reason, unaided by experience, is continually thrown into chasms of perplexities. Claims that run contrary to common sense are those in most need of the rehabilitative effects of experience.
Academic skepticism, in contrast to antecedent skepticism, is derived doubt. One example is skepticism about external objects. How can one know, for example, that objects not only exist, but are also the cause of one's ideas of them? After all, one only experiences various impressions, for example, of color or shape, never the object directly, or as a whole. Moreover, it's unclear what to make of an object's status when it is not perceived. Sensations, therefore, do not provide adequate evidence for one's belief in the existence of external objects.
Deriving objects from beliefs about a divinity is also dubious, since there are already skeptical arguments about the existence of God. Is one to conclude, like Berkeley, that objects are merely mental entities? Hume's response is that radical skepticism is impractical.
"Mitigated" skepticism or "academical" philosophy is Hume's preferred approach. It reflects modesty and careful scrutiny, which are features of any impartial seekers of truth. The sort of demonstrations found in mathematics are not available in a science of mind, guided as it is by matters of fact, or moral philosophy.
What Hume thinks his empiricism yields is the rejection of any claims and theories that cannot be verified either by relations of ideas—specifically mathematics—or matters of fact. These, he maintains, reflect "nothing but sophistry and illusion." A Sophist was an ancient Greek teacher, one with a reputation not for seeking truth, but for helping students learn how to speak well and win arguments.
Hume is not the sort of skeptic he portrays Descartes to be. Indeed, this is part of the point of juxtaposing the universal doubter with the consequential doubter. The purpose of doubt is not to derail belief, but to reorient inquiry properly. The aim, after all, is truth.
In its extreme form, consequential doubt, like universal doubt, leads to a sort of inertia. Hume is more concerned, however, with enlisting skepticism to reign in the excesses of metaphysical claims made using relations of ideas and matters of fact. Such claims include the existence of God.
It is worth pointing out that, in addition to his criticism of Descartes, Hume is also thinking about Locke and Berkeley in his discussion of the problem of an external world. Locke argues, in part, that ideas can be traced to objects that exist independently of any perceiver. There are, he asserts, powers in objects to produce ideas, such as sweet, hot, rough, and so forth. These powers are an object's secondary qualities. Its primary qualities are those that cannot be divested from the object without that object ceasing to be; figure, extension, solidity, motion, and number are inherent in the object.
Berkeley's main criticism of Locke's view is that it leads to skepticism about the external world. If Locke is correct, to perceive an object is not necessarily to have an accurate picture of it; there is what is perceived—its sensible or secondary qualities—and then there is the object as it really is, that is, the object as defined by its primary qualities. All a human can perceive is ideas, which Berkeley defines as collections of sensations. An apple, for example, is the collections of sensations perceived—the smoothness, roundness, crunchiness, juiciness, and so forth. To assert that there is something else besides sets up a world in which what is perceived is not what really is.
For Hume, Berkeley's critique is an extreme form of consequential doubt. In its place, he wants, once again, to focus one's thinking on the proportional evidence for the view in question. So, although in principle he arguably agrees with Berkeley's view about perception, his conclusion is more conservative. It is a suspension of judgment about the metaphysical status of the objects themselves.
Hume's skepticism pervades his writing on knowledge, reality, and religion. He is particularly interested in reassigning a variety of functions typically associated with reason to nonrational capacities, mechanisms, and states. For example, causal inference is not a rational process, but a psychological disposition.
Healthy skepticism consists in an attitude of open-mindedness but also a commitment to limiting inquiries "to such subjects as are best adapted to the narrow capacity of human understanding." Concerned as he is with avoiding claims that cannot be substantiated by experience, Hume may rightly be declared less a skeptic of the doubting variety and more of an intellectual conservative in matters of knowledge.