Front Page Titles (by Subject) § 3.: Of Extension - Selections from the Scottish Philosophy of Common Sense
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§ 3.: Of Extension - 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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It is further to be observed, that hardness and softness, roughness and smoothness, figure and motion, do all suppose extension, and cannot be conceived without it; yet, I think it must, on the other hand, be allowed that, if we had never felt any thing hard or soft, rough or smooth, figured or moved, we should never have had a conception of extension; so that, as there is good ground to believe that the notion of extension could not be prior to that of other primary qualities, so it is certain that it could not be posterior to the notion of any of them, being necessarily implied in them all.
Extension, therefore, seems to be a quality suggested to us, by the very same sensations which suggest the other qualities above mentioned. When I grasp a ball in my hand, I perceive it at once hard, figured, and extended. The feeling is very simple, and hath not the least resemblance to any quality of body. Yet it suggests to us three primary qualities perfectly distinct from one another, as well as from the sensation which indicates them. When I move my hand along the table, the feeling is so simple that I find it difficult to distinguish it into things of different natures; yet, it immediately suggests hardness, smoothness, extension, and motion—things of very different natures, and all of them as distinctly understood as the feeling which suggests them.
We are commonly told by philosophers, that we get the idea of extension by feeling along the extremities of a body, as if there was no manner of difficulty in the matter. I have sought, with great pains, I confess, to find out how this idea can be got by feeling; but I have sought in vain. Yet it is one of the clearest and most distinct notions we have; nor is there anything whatsoever about which the human understanding can carry on so many long and demonstrative trains of reasoning.
The notion of extension is so familiar to us from infancy, and so constantly obtruded by everything we see and feel, that we are apt to think it obvious how it comes into the mind; but upon a narrower examination we shall find it utterly inexplicable. It is true we have feelings of touch, which every moment present extension to the mind; but how they come to do so, is the question; for those feelings do no more resemble extension than they resemble justice or courage—nor can the existence of extended things be inferred from those feelings by any rules of reasoning; so that the feelings we have by touch, can neither explain how we get the notion, nor how we come by the belief of extended things.
What hath imposed upon philosophers in this matter is, that the feelings of touch, which suggest primary qualities, have no names, nor are they ever reflected upon. They pass through the mind instantaneously, and serve only to introduce the notion and belief of external things, which, by our constitution, are connected with them. They are natural signs, and the mind immediately passes to the thing signified, without making the least reflection upon the sign, or observing that there was any such thing. Hence it hath always been taken for granted, that the ideas of extension, figure, and motion, are ideas of sensation, which enter into the mind by the sense of touch, in the same manner as the sensation of sound and smell do by the ear and nose. The sensations of touch are so connected, by our constitution, with the notions of extension, figure, and motion, that philosophers have mistaken the one for the other, and never have been able to discern that they were not only distinct things, but altogether unlike. However, if we will reason distinctly upon this subject, we ought to give names to those feelings of touch; we must accustom ourselves to attend to them, and to reflect upon them, that we may be able to disjoin them from, and to compare them with, the qualities signified or suggested by them.
The habit of doing this is not to be attained without pains and practice; and till a man hath acquired this habit, it will be impossible for him to think distinctly, or to judge right, upon this subject.
Let a man press his hand against the table—he feels it hard. But what is the meaning of this?—The meaning undoubtedly is, that he hath a certain feeling of touch, from which he concludes, without any reasoning, or comparing ideas, that there is something external really existing, whose parts stick so firmly together, that they cannot be displaced without considerable force.
There is here a feeling, and a conclusion drawn from it, or some way suggested by it. In order to compare these, we must view them separately, and then consider by what tie they are connected, and wherein they resemble one another. The hardness of the table is the conclusion, the feeling is the medium by which we are led to that conclusion. Let a man attend distinctly to this medium, and to the conclusion, and he will perceive them to be as unlike as any two things in nature. The one is a sensation of the mind, which can have no existence but in a sentient being; nor can it exist one moment longer than it is felt; the other is in the table, and we conclude, without any difficulty, that it was in the table before it was felt, and continues after the feeling is over. The one implies no kind of extension, nor parts, nor cohesion; the other implies all these. Both, indeed, admit of degrees, and the feeling, beyond a certain degree, is a species of pain; but adamantine hardness does not imply the least pain.
And as the feeling hath no similitude to hardness, so neither can our reason perceive the least tie or connection between them; nor will the logician ever be able to shew a reason why we should conclude hardness from this feeling, rather than softness, or any other quality whatsoever. But, in reality, all mankind are led by their constitution to conclude hardness from this feeling.
The sensation of heat, and the sensation we have by pressing a hard body, are equally feelings; nor can we, by reasoning, draw any conclusion from the one but what may be drawn from the other: but, by our constitution, we conclude from the first an obscure or occult quality, of which we have only this relative conception, that it is something adapted to raise in us the sensation of heat; from the second, we conclude a quality of which we have a clear and distinct conception—to wit, the hardness of the body.1
[1 ]Ibid., pp. 123-125.