In a word, the dictates of common sense are, in respect to human knowledge in general, what the axioms of geometry are in respect to mathematics: on the supposition that those axioms are false or dubious, all mathematical reasoning falls to the ground; and on the supposition that the dictates of common sense are erroneous and deceitful, all science, truth, and virtue, are vain. (p. 122)
To the end of Part I he has argued mostly for the first part, that the dictates of common sense provide the first principles for human knowledge. Part II will be mostly concerned with the second: that a proper view of common sense, and the first principles it provides, is absolutely essential for "science, truth, and virtue" and that serious consideration of common sense itself shows where the skeptics go wrong. Again, it is always worth keeping in mind that Beattie's aims throughout the Essay are pro-science and pro-ethics: he opposes skeptics like Hume because he thinks they are a disaster on both points.
In Part I he argued generally that each field of inquiry had its own first principles, recognized by common sense. He begins Part II by arguing that when we look at the work of actual mathematicians and scientists, we find that they too recognize in some way the distinction between common sense and reason, giving the priority to common sense, and that this in fact structures their investigations; and that skepticism would lead to the destruction of all such healthy inquiry. For instance, geometry was able to advance by taking certain things as axiomatic and obvious. At this point he engages in an elaborate parody of Hume, pulling sentences from several different parts of Hume's Treatise, in which he has the skeptic argue that anything obvious is just prejudice, that philosophers must always be doubting, even about their doubts, that practice is irrelevant to the question, that believing anything with certainty is foolish, that he is certain that human beings are not certain of anything. (This provides the occasion for one of Beattie's incomparable insults of Hume. In a footnote he concedes that it is not entirely fair to build a parody using sentences from such widely dispersed passages, and says that he would gladly attribute Hume's expressions to inadvertence. But, he says, "then I must impute the whole system to the same cause" (p. 146). The point is that the skeptic is out to guarantee that the actual operation of mathematical inquiry, in which one traces things back to things that seem obvious and build on them, never gets started, by taking out anything that's obvious. And while not every science has the certainty of mathematics, it is unreasonable in any science to reject out of hand whatever is obvious, or to treat obviousness as a mere prejudice. We inquire by taking as certain those things that seem intrinsically obvious and whose apparent obviousness cannot be affected by reasoning.
As with mathematics, so with natural philosophy, i.e., the natural sciences, which draw on two sources, the evident principles of mathematics and the evident principles of sense. All natural philosophy traces itself back either to the evidence of mathematical principles or to the evidence of sensation; at some fundamental level these must be trusted or we cannot say that natural philosophy in its investigations is really uncovering anything at all. This is true, even given the fact that we recognize our senses to be sometimes misleading; for most of these cases boil down either to jumping to conclusions in our interpretation of sense, which can be discovered by closer analysis, or to effects of particular conditions, which are discovered by sense. The last is rather important, since optical illusions and the like are regularly brought forward as reasons why we can't trust our senses. But how do we know that they are illusions at all? Because we judge our senses under tricky conditions by the standard of our senses under more certain conditions, and because we trust our senses at a more basic level. This is the way inquiry works. In inquiry there will always be tricky cases, unusual circumstances, etc., but we get through these by trusting more fundamental issues. We recognize downstream sensory illusions as illusions by trusting our senses to give us information upstream from those illusions, by having the confidence that the sensory context in which they are recognized to be illusions is itself trustworthy. What we are often really pointing to is simply the fact that our senses don't cover everything with equal certainty; we are showing that our senses aren't complete and perfect, not that they are fundamentally deceptive, or that they cannot be trusted. And it is important to note that Beattie includes internal senses as well as external senses. There may be situations where introspection can be fooled, but it's only possible to recognize this by trusting some kinds of introspection as certain and obvious (just as one very obvious example, our introspection-based certainty that we are capable of introspection). Likewise, there may be situations where our sense of beauty or moral sense has difficulty finding purchase, but this does not show that there are no circumstances in which they provide full certainty. The thing of it is, too, that in all these fields, taking some things as obvious and certain is not detrimental to the inquiry in any way: you can still investigate whatever you please. It's just that you can't get anywhere in an investigation without taking some things as obvious, so no matter what kind of investigation you may have, it will presuppose some evident first principles somewhere.
One of the things Beattie is angling at in all of this is that if this is true of mathematics and natural philosophy, it will also be true in the philosophy of human nature and the moral sciences: if you want progress on the subject of human nature, you can't go about it in the way Hume does, by taking a skeptical approach, but must take some things as evident first principles, so that those principles can serve as the template for your inquiries and investigations, and so that the certainty of those first principles can be communicated, so to speak, to your conclusions, to make your conclusions knowledge. All inquiry, of whatever kind, is grounded, guided, and evaluated by the standard of common sense. If it turns out that you have some doubt about whether something really is a dictate of common sense, you test it out by (1) thinking it through very carefully and determining whether it seems certain, in such a way that you are naturally forced to trust it; (2) investigating whether it is consistent under different conditions; (3) acting on it to see whether you get good results; (4)
determining whether it is consistent with other dictates of common sense; and (5) seeing whether others seem determined by their nature to accept it. If it meets all these criteria, it is the product of a well-informed sense, and can be taken as such; and, while such dictates can be divided into different kinds of certainty (mathematical, moral, sufficient for most practical purposes) according to the universality of their scope and their force as principles, there's a sense in which evident is evident and certain is certain. That you can't get mathematical certainty from the senses doesn't mean that the senses aren't able to give you anything evidently and obviously true.
Skepticism, of course, is opposed to this whole approach. I will not here go into Beattie's account of the history of modern skepticism; it's a pretty standard kind of history, starting with Descartes and ending with Hume, and he is largely drawing it from Reid. There are some interesting aspects to it, though, such as his comparison of modern skeptics to ancient sophists; one notable feature of which is that he gives the preference to Xenophon's account of Socrates rather than Plato's. After giving this historical run-down he looks at two philosophical topics in particular: skeptical arguments that matter does not exist, which he uses to exhibit the skeptical approach to the external senses, and skeptical arguments that there is no free will, which he uses to exhibit the skeptical approach to the internal senses. In truth, we believe that there is an external world independent of us, regardless of the arguments of Berkeley or anyone else; and why? Because it is a dictate of common sense. A similar course of reasoning arises when discussing free will, although the latter discussion is more interesting in part because Beattie makes two interesting detours (which, however, ultimately contribute to his argument), one to argue that Hume's account of causation is arbitrary and inconsistent with common sense, and another brief one to argue that free will is found implicitly, and sometimes explicitly, in a wide variety of contexts, such as in the works of Homer and Virgil. Beattie's arguments are rather long, but we can simply summarize them here as saying that they are applications of the kinds of arguments we have already seen.
In none of this is he entirely fair to some of the people involved -- Malebranche and Berkeley in particular have more sophisticated and less absurd views than are attributed to them. However, in none of this is Beattie being malicious, either, since he is simply building on standard interpretations of the figures involved. (Indeed, the interpretation of Berkeley, although not quite fair to Berkeley, is still the standard interpretation.) And, it must be pointed out, that Beattie is chiefly concerned with influence: whatever they may have meant (he complains regularly about how skeptics never say something clearly if they can say it obscurely), they are still being used to argue for these skeptical positions that are destruction of "science, truth, and virtue."
And this, of course, is what concerns Beattie here. Beattie acknowledges that his argument is quite roundabout, but he wants to drive home the fact that skeptics like Hume are not benign investigators but advocating arbitrary principles that are destructive of all serious investigation. He then goes on in Part III to wrap up some miscellaneous issues, like freedom of inquiry and the nature of metaphysics. We won't discuss most of these, but one of this miscellaneous issues, arising from his discussion of the degeneration of the moral sciences, is famous in its own right, namely, Beattie's defense of egalitarianism and attack on Hume's racism. So we'll look at that in the next post.