Front Page Titles (by Subject) § 2.: Of Natural Signs - Selections from the Scottish Philosophy of Common Sense
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§ 2.: Of Natural Signs - Thomas Reid, Selections from the Scottish Philosophy of Common Sense 
Selections from the Scottish Philosophy of Common Sense, edited, with an introduction by G.A. Johnston (Chicago: Open Court, 1915).
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Of Natural Signs
As in artificial signs there is often neither similitude between the sign and thing signified, nor any connection that arises necessarily from the nature of the things, so it is also in natural signs. The word gold has no similitude to the substance signified by it; nor is it in its own nature more fit to signify this than any other substance; yet, by habit and custom, it suggests this and no other. In like manner, a sensation of touch suggests hardness, although it hath neither similitude to hardness, nor, as far as we can perceive, any necessary connection with it. The difference betwixt these two signs lies only in this—that, in the first, the suggestion is the effect of habit and custom; in the second, it is not the effect of habit, but of the original constitution of our minds.
It appears evident from what hath been said on the subject of language, that there are natural signs as well as artificial; and particularly, that the thoughts, purposes, and dispositions of the mind, have their natural signs in the features of the face, the modulation of the voice, and the motion and attitude of the body: that, without a natural knowledge of the connection between these signs and the things signified by them, language could never have been invented and established among men: and, that the fine arts are all founded upon this connection, which we may call the natural language of mankind. It is now proper to observe, that there are different orders of natural signs, and to point out the different classes into which they may be distinguished, that we may more distinctly conceive the relation between our sensations and the things they suggest, and what we mean by calling sensations signs of external things.
The first class of natural signs comprehends those whose connection with the thing signified is established by nature, but discovered only by experience. The whole of genuine philosophy consists in discovering such connections, and reducing them to general rules. The great Lord Verulam had a perfect comprehension of this, when he called it an interpretation of nature. No man ever more distinctly understood or happily expressed the nature and foundation of the philosophical art. What is all we know of mechanics, astronomy, and optics, but connections established by nature, and discovered by experience or observation, and consequences deduced from them? All the knowledge we have in agriculture, gardening, chemistry, and medicine, is built upon the same foundation. And if ever our philosophy concerning the human mind is carried so far as to deserve the name of science, which ought never to be despaired of, it must be by observing facts, reducing them to general rules, and drawing just conclusions from them. What we commonly call natural causes might, with more propriety, be called natural signs, and what we call effects, the things signified. The causes have no proper efficiency or causality, as far as we know; and all we can certainly affirm is, that nature hath established a constant conjunction between them and the things called their effects; and hath given to mankind a disposition to observe those connections, to confide in their continuance, and to make use of them for the improvement of our knowledge, and increase of our power.
A second class is that wherein the connection between the sign and thing signified, is not only established by nature, but discovered to us by a natural principle, without reasoning or experience. Of this kind are the natural signs of human thoughts, purposes, and desires, which have been already mentioned as the natural language of mankind. An infant may be put into a fright by an angry countenance, and soothed again by smiles and blandishments. A child that has a good musical ear, may be put to sleep or to dance, may be made merry or sorrowful, by the modulation of musical sounds. The principles of all the fine arts, and of what we call a fine taste, may be resolved into connections of this kind. A fine taste may be improved by reasoning and experience; but if the first principles of it were not planted in our minds by nature, it could never be acquired. Nay, we have already made it appear, that a great part of this knowledge which we have by nature, is lost by the disuse of natural signs, and the substitution of artificial in their place.
A third class of natural signs comprehends those which, though we never before had any notion or conception of the thing signified, do suggest it, or 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. I shewed formerly, that our sensations suggest to us a sentient being or mind to which they belong—a being which hath a permanent existence, although the sensations are transient and of short duration—a being which is still the same, while its sensations and other operations are varied ten thousand ways—a being which hath the same relation to all that infinite variety of thoughts, purposes, actions, affections, enjoyments, and sufferings, which we are conscious of, or can remember. The conception of a mind is neither an idea of sensation nor of reflection; for it is neither like any of our sensations, nor like anything we are conscious of. The first conception of it, as well as the belief of it, and of the common relation it bears to all that we are conscious of, or remember, is suggested to every thinking being, we do not know how.
The notion of hardness in bodies, as well as the belief of it, are got in a similar manner; being, by an original principle of our nature, annexed to that sensation which we have when we feel a hard body. And so naturally and necessarily does the sensation convey the notion and belief of hardness, that hitherto they have been confounded by the most acute inquirers into the principles of human nature, although they appear, upon accurate reflection, not only to be different things, but as unlike as pain is to the point of a sword.
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.
I take it for granted, that the notion of hardness, and the belief of it, is first got by means of that particular sensation which, as far back as we can remember, does invariably suggest it; and that, if we had never had such a feeling, we should never have had any notion of hardness. I think it is evident, that we cannot, by reasoning from our sensations, collect the existence of bodies at all, far less any of their qualities. This hath been proved by unanswerable arguments by the Bishop of Cloyne, and by the author of the Treatise of Human Nature. It appears as evident that this connection between our sensations and the conception and belief of external existences cannot be produced by habit, experience, education, or any principle of human nature that hath been admitted by philosophers. At the same time, it is a fact that such sensations are invariably connected with the conception and belief of external existences. Hence, by all rules of just reasoning, we must conclude, that this connection is the effect of our constitution, and ought to be considered as an original principle of human nature, till we find some more general principle into which it may be resolved.1
[1 ]Ibid., pp. 121-122.