The Treatise of Human Nature has given David Hume a reputation for being the arch-skeptic. After beginning his work with the goal of detailing a "science of man" he proceeds through the argument to undercut our confidence in the reliability of the senses, the objectivity of our notions of space and time, the reality of causal laws, and the rationality of our beliefs. By the end of Book I, he seems to have devastated human knowledge and beliefs, leaving us with the mere shell of skepticism. But then, paradoxically, he proceeds to write two more books in the Treatise as well as many other works in which these considerations appear to be largely ignored. So the question becomes: how seriously should we take Hume's skepticism?
Let's start at the end of Book I Part iv of the Treatise. Having just argued against the ability of reason to arrive at certain knowledge (the result of the application of reason is the “total extinction of belief and evidence”), against the reliability of the senses, and even against the idea of a stable self, Hume despairs over the limits of the understanding to arrive at true knowledge. Our faculties are irreparably deficient for the job to which we apply them in philosophical reasoning. This despair applies equally to Hume’s own views as well—he can offer no reason for his conviction that he has reasoned correctly in abandoning the received view of the understanding, meaning his whole argument to this point is subject to doubt. Common life takes no note of these deficiencies, but instead operates by custom and habit. We act as if cause and effect are objective features in the world, as if we are a stable, unitary self, and as if our psychological certainty about the existence of the world is justified, but these are merely the illusory effects of custom. The issue, though, is the extent to which we should submit to these illusions (T, I.iv.7, 263-7, page numbers refer to Selby-Bigge/Nidditch edition).
At this point in the Treatise Hume offers only one way out of the skeptical impasse. “Fancy” alone saves us from outright skepticism. Our choice is between “a false reason and none at all,” but ultimately there is no choice. No matter how deeply involved in his philosophical contemplation Hume gets, nature “cures [him] of this philosophical melancholy and delirium.” Our natural tendencies will overpower a reasoned conviction that leads to total skepticism (T, I.iv.7, 267-70).
It seems as if the natural reading of the Treatise should be that the outcome of Hume’s philosophy is total skepticism, leading us to the conclusion that there are no warranted beliefs and that we ought to simply abandon the pursuit of truth and learn to live with the irrationality of daily life. But I think there are some passages of the Treatise, both in this section and others that open the possibility to a reading that is more in line with the common “skeptical realist” interpretation of Hume’s later thought (especially the First Enquiry). The skeptical realist view yields a reading of Hume as concerned with showing the limits of reason, partly by showing its fallibility, but more by demonstrating that reason itself is grounded on natural processes and is thus subject to empirical investigation. Ultimately, I think that the extreme skeptical reading of Hume mischaracterizes the thrust of his argument in the Treatise and the overall purpose of the work. If we view Hume’s project as a descriptive rather than a normative work, the significance of the specter of skepticism is reduced considerably.
First, despite the fact that reason leads us to skeptical conclusions, Hume acknowledges in the concluding section of Book I that such a conclusion is purely intellectual, and can have no real effect on the day to day lives (and reasonings) of people. Although reason cannot justify this setting aside of skeptical considerations, it is simply not possible to live life in such a way as to accommodate extreme skepticism. But I don’t think that Hume takes nature’s conquest of philosophical delirium to be a Pyrrhic victory. Hume’s project in the Treatise has not been one of justification; from the introduction forward, his goal has been an empirical description of human nature. His goal is a science of man, modeled on the other empirical sciences. He then proceeds to catalog the processes by which we obtain simple ideas and form complex ideas, how those ideas are associated with one another, and how we get notions like cause and effect. None of this project has been normative, so the scope of the significance of the problems of justification in Book I Part IV seems limited. Add to this the fact that Hume continues his descriptive project in Books II and III without offering arguments to overcome the skeptical considerations, and it seems clear that Hume didn’t consider these to be debilitating problems for his philosophical interests.
Second, Hume has already outlined his theory of belief formation, which ties the idea of a belief to the force or vivacity of the idea. Beliefs are lively and forceful ideas (94-5), which have natural causes (98-9). Even our beliefs about the probability of a particular event are formed involuntarily through natural mechanisms (T I.iii.12). It would seem strange for Hume to detail a descriptive account of involuntary belief formation (which does not include reason or philosophical justification as a component) and then to argue that reason should override by some unknown mechanism the natural processes by which we form beliefs.
Third, there are places in the Treatise where Hume makes the distinction between what we know and how we know it on the one hand, and the actual makeup of the world on the other, leading to the possibility of reading him as a skeptical realist. For one example, take his discussion of necessary connection at I.iii.14. Hume uses his empiricist theory to provide a descriptive account of custom as the source of our idea of power and necessary connection, but then he frames this discussion in terms of the limits of his claim. Hume’s claim is not that there is no power in the world; instead he is claiming only that whatever power exists in the world is beyond our acquaintance or knowledge. His point is not the metaphysical point that there is no necessary connection in the world, but only the epistemological point that if there is such a real relation it is not the source of our idea of necessary connection. In fact, even the causal relations that obtain between the impressions and ideas that make up our experiences are as mysterious to us as those of the world, and are only known to us by custom through experience (168-9).
These three points serve to at least call into question the picture of Hume as an unrepentant, extreme skeptic. Ultimately, the skeptical sections of the Treatise might be more productively read as a reductio argument to demonstrate the problems reason gets into when it oversteps its bounds. Skeptical considerations serve to mitigate the claims of reason and philosophy, and force us from unwarranted normative theories into descriptive accounts that can teach us about ourselves and about our patterns of inference. In this way that Hume can still hope to “contribute a little to the advancement of knowledge” (273), despite the skeptical problems he’s just enumerated.