Natural signs Reid takes to differ from artificial signs in the fact that the suggestion (of what is signified by the sign) is in the case of artificial signs based on habit and custom; but in the case of natural signs it is based on the way our minds are constituted. He identifies three classes of natural signs.
In the first class are signs whose connection to what they signify is established by nature and discovered through experience. Science is based on the discovery of such signs -- causes as the signs of effects and so forth -- and so Reid uses Bacon's phrase interpretation of nature to describe this class.
The second class of natural signs is the class of signs that are not only established by nature but are known by some basic principle of our minds when we experience it. These Reid calls the natural language of mankind because they underlie all our communications. An infant, for instance, is frightened by an angry face and soothed by a smiling one. According to Reid, the ultimate principles of taste are all found here; taste is refined by reasoning and experience, but without the basic principles we'd have no taste to refine at all. The ability to appreciate this or that would simply not be part of the world as we experience.
The third class of natural signs consists of cases where the thing signified is suggested by the sign even though we have never actually experienced the thing signified (except through the sign, of course). What is built into us is not merely the ability to recognize the connection between sign and signified, as in the second class, but also the ability to know the signified at all. In the second class we can experience both sign and signified and know the connection by our natural ability. But in the case of the third class we only know the signified because the signs "conjure it up, as it were by a natural kind of magic, and at once give us a conception, and create a belief of it." For instance, our perceptions, thoughts, and desires suggest several features of the mind that has them even though we have no experience of our minds except through them: they suggest, for instance, that it has a continuing existence that underlies all of these thoughts. We have no sensation of this continuing existence, so it is not something we experience at all except insofar as we find it suggested by what we do directly experience. And the same is true, of course, when we extend the question to other minds than our own. Likewise, Reid suggests that hardness is another signified that we know only through its signs: we have a certain feeling of bodies and this suggests immediately something about their constitution, so immediately, in fact, that we easily confound the two.
Reid sums up this classification in an interesting passage (Inquiry, Chapter 5, Section 3):
It may be observed, that, as the first class of natural signs I have mentioned is the foundation of true philosophy, and the second the foundation of the fine arts, or of taste--so the last is the foundation of common sense--a part of human nature which hath never been explained.