There are certain kinds of propositions which, when understood, receive the immediate assent of our minds, while their contraries are immediately disbelieved; and such is our assent that, at least for the most part, we take ourselves to be as certain of these things as we would be if we were "perfectly acquainted with all nature, in all its parts, and in all its laws" (p. 33). If I take a proposition like, "I exist," I assent to it immediately; I disbelieve its opposite immediately; and my certainty in this proposition is so great that I judge that I would still believe that I exist even if I knew infinitely more about the universe than I do. This is a simple assent to it as true; when I say, "I exist," I take it to be in the nature of things that I exist, or, as Beattie prefers to put it, that the proposition expresses something conformable to the nature of things. But what do we mean when we talk about the nature of things? That there is something in our minds that makes us think that things exist deteriminately according to laws which determine them, and that the proposition agrees with those laws. Note the very important entrance of this 'something in our mind that makes us think'. We have minds, in other words, that tend to think in terms of truth, in terms of what is really the case, and we could not really avoid doing so even if we tried. On this basis, Beattie says, "truth" means that which the constitution of our nature determines us to believe, and "falsehood" means that which the constitution of our nature determines us to disbelieve.
This, then, is the Common Sense theory of truth. It is important to make a distinction. It does not say that truth is whatever we think it is; it says, rather, that truth is whatever our natures compel us to believe. Falsehood is not whatever we think it is; rather, falsehood is whatever the set-up of our minds themselves forces us to disbelieve. We can believe many false things; but if our minds themselves are set up so that we cannot actually believe something false, there is no good pretending that we can make sense of the notion that they might actually be false. Likewise, if our minds are set up so that they always, willy nilly, disbelieve something, there's no good pretending that this might actually be true. If you believe something, you regard it as true; if you can't help but believe something, you can't help but regard it as true, whatever you may say. And it comes to the same thing if, instead of talking about truth, we talk about certain truth and probable truth.
We can, however, make an important distinction between two different kinds of truths: those that are intuitively perceived to be true and those that are perceived to be true in consequence of a line of proof. It makes sense to distinguish these two different abilities by two different names; and Common Sense philosophers like Beattie call the first common sense and the second reason. Beattie notes that, although terms are not always used the same way, the distinction itself is an old one, and is found in the ancient geometers and in Aristotle under a different terminology; and also that we must be very careful not to be misled by different senses of these terms, since both terms are used in a wide range of very different contexts to mean somewhat different things. "Common sense," as used here, means
...that power of the mind which perceives truth, or commands belief, not by progressive argumentation, but by an instantaneous, instinctive, and irresistible impulse; derived neither from education nor from habit, but from nature; acting independently on our will, whenever its object is presented, according to an established law, and therefore not improperly called Sense; and acting in a familiar manner upon all, or at least upon a great majority of mankind, and therefore properly called Common Sense. (p. 45)
For the reason why it is called a "Sense" he refers the reader to Gerard's Essay on Taste, another important work in the Scottish Common Sense tradition. Suffice it to say here that this is a broader use of the term than when simply used of the five senses, and is closely related to the sense it has when we talk about a 'sense of humor' or 'sense of proportion' or 'sense of beauty'. He then argues that reason and common sense must be distinct abilities, neither resolvable into the other.
(1) We are conscious that the two feel different. When we believe something true because of a reasoned investigation, we are sensible of the connection of this belief with our ability to assign a reason. When, however, we believe something true because it is an intuitive principle, we are sensible of the connection of this belief with the fact that it is natural to believe the principle and unnatural not to believe it.
(2) While common sense and reason are often connected, we find no obvious necessary connection between them. In fact, says Beattie, there are certain cases in which we reason with a lack of common sense: dreams, for instance, or madness. As Beattie says, someone who thinks himself made of glass can reason very accurately about what follows from this. (It's noteworthy that we find this same line of reasoning, occasionally using the same example, in Chesterton's "The Maniac"; there is nothing, as far as I am aware, to connect the two, but it's an interesting commonality.) We don't have to go madness, though, to see the same thing. People who read a great many polemical works often develop their reasoning faculty to a considerable degree while allowing their common sense to remain in a very defective state; they argue without understanding, but still often manage to argue to great effect. This is also connected to the third difference.
(3) Education and culture can do an immense amount to improve our reason. Common sense is much less in our power to cultivate: "To teach the art of reasoning, or rather of wrangling, is easy; but it is impossible to teach common sense to one who wants it" (p. 47). You can teach such people first principles, make them memorize it, but the sense of their truth will always be beyond your power to impart. Likewise, different people are born with different strengths of common sense; some can see mathematical principles at once, some never will. And we find, too, that of two people who accept the same first principle, one may hold it lightly, being easily persuaded out of it, while the other may hold it very deeply, being so sensible of its truth that it has an immense impact on life and reasoning. This is to a great extent simply a matter of temperament. Beattie does not deny that common sense can be developed; like all capabilities, it is developed by being used. Likewise, not using it leads to its weakening. (Being a good Calvinist, his example is a Catholic who, led by false religion, simply accepts what the priest says regardless of what sense it makes.) An atmosphere with freedom of inquiry will be better suited for the development of common sense than any other. But common sense is massively more difficult to develop than reason, and can only be developed to a certain extent.
(4) Something like the distinction is recognized by pretty much everyone. People will sometimes admit that an argument is extraordinarily good, indeed, beyond their ability to refute, but still utterly unconvincing. It really doesn't matter how good Berkeley's arguments against the material world may be; people who cannot refute him still come away unconvinced. This is injurious to philosophical pride; people inclined to argue like to dismiss rejections of argument not based on refutation; but none of this makes it any less true that a person can be faced with an apparently irrefutable argument and not be swayed by it at all because the conclusion seems so obviously wrong.
Thus the two, reason and common sense, are distinct. People are very reluctant, however, to admit that they may have cognitive instincts of the sort that is labeled by common sense. Beattie seems to think that this is due to pride: animals have instincts, we have reason. But, in fact, Beattie wants to argue, the opposite is true: we have more and better instincts than animals and if we observed ourselves carefully rather than interpreting ourselves through a preconception of ourselves, we would see these impulses of nature working entirely independently of reason. Moreover, important as reason is, we must not overlook the extraordinary ways in which it is perverted and abused to sophistical ends; it has its weaknesses as well as its strengths. And this gets us precisely into seeing the necessity of common sense. Berkeley and Hume come to you and say that everything around you is really just in your mind. If you think this contrary to common sense, this just means your common sense is wrong; it is proved wrong by argument. But, says Beattie, if my common sense -- and remember, we are not talking about just 'commonly accepted truths', but the kind of assent deriving directly from the constitution of our nature -- is mistaken, how could I possibly correct the mistake? Nothing reason can throw at me is going to be clearer, stronger, or more obvious than what common sense is telling me. Common sense is about what we are constrained to believe; we can't unconstrain ourselves just because someone says we should. Even if we set this aside, though, even if we assumed, contrary to actual experience, that reason were just as forceful as common sense, what good would this do? We would have two powerful cognitive faculties each telling us something inconsistent with what the other is telling us. And why would we assume that reason is more likely to be right? Because reason says so? But the whole point is that common sense is saying that reason has actually come to the wrong answer. Which shall we believe? Either they both may be inconsistent, and then we are in miserable straits when they conflict, as we know they can, or one must be superior to the other.
One reason you might give for saying that reason should be superior to common sense is the claim that all philosophy, all reasonable inquiry, should begin in doubt, and nothing believed without proof. This would certainly require the superiority of reason; reason would be the ultimate standard of truth, and common sense would, at best, simply have to submit to reason. But this turns out to be an untenable position -- we can show that it is unreasonable to start with doubt, and that all inquiry goes back to first principles. And this is what Beattie will argue in the next chapter.