The Essays of Adam Smith

by Adam Smith

Every such body must likewise be conceived as capable both of motion and of rest; both of altering its situation with regard to other surrounding bodies, and of remaining in the same situation. That bodies of small or moderate bulk, are capable of both motion and rest we have constant experience. Great masses, perhaps, are according to the ordinary habits of the imagination, supposed to be more fitted for rest than for motion. Provided a sufficient force could be applied, however, we have no difficulty in conceiving that the greatest and most unwieldy masses might be made capable of motion. Philosophy teaches us, (and by reasons too to which it is scarcely possible to 441 refuse our assent,) that the earth itself, and bodies much larger than the earth, are not only movable, but are at all times actually in motion, and continually altering their situation, in respect to other surrounding bodies, with a rapidity that almost passes all human comprehension. In the system of the universe, at least according to the imperfect notions which we have hitherto been able to attain concerning it, the great difficulty seems to be, not to find the most enormous masses in motion, but to find the smallest particle of matter that is perfectly at rest with regard to all other surrounding bodies.

These four qualities, or attributes of extension, divisibility, figure, and mobility, or the capacity of motion or rest, seem necessarily involved in the idea or conception of a solid substance. They are, in reality, inseparable from that idea or conception, and the solid substance cannot possibly be conceived to exist without them. No other qualities or attributes seem to be involved, in the same manner, in this our idea or conception of solidity. It would, however, be rash from thence to conclude that the solid substance can, as such, possess no other qualities or attributes. This rash conclusion, notwithstanding, has been not only drawn, but insisted upon, as an axiom of indubitable certainty, by philosophers of very eminent reputation.

Of these external and resisting substances, some yield easily, and change their figure, at least in some degree, in consequence of the pressure of our hand: others neither yield nor change their figure, in any respect, in consequence of the utmost pressure which our hand alone is capable of giving them. The former we call soft, the latter hard, bodies. In some bodies the parts are so very easily separable, that they not only yield to a very moderate pressure, but easily receive the pressing body within them, and without much resistance allow it to traverse their extent in every possible direction. These are called Fluid, in contradistinction to those of which the parts not being so easily separable, are upon that account peculiarly called Solid Bodies; as if they possessed, in a more distinct and perceptible manner, the characteristical quality of solidity or the power of resistance. Water, however (one of the fluids with which we are most familiar), when confined on all sides (as in a hollow globe of metal, which is first filled with it, and then sealed hermetically), has been found to resist pressure as much as the very hardest, or what we commonly call the most solid bodies.

Some fluids yield so very easily to the slightest pressure, that upon, ordinary occasions we are scarcely sensible of their resistance; and are upon that account little disposed to conceive them as bodies, or as things capable of pressure and resistance. There was a time, as we may learn from Aristotle and Lucretius, when it was supposed to require some degree of philosophy to demonstrate that air was a real solid body, or capable of pressure and resistance. What, in ancient 442 times, and in vulgar apprehensions, was supposed to be doubtful with regard to air, still continues to be so with regard to light, of which the rays, however condensed or concentrated, have never appeared capable of making the smallest resistance to the motion of other bodies, the characteristical power or quality of what are called bodies, or solid substances. Some philosophers accordingly doubt, and some even deny, that light is a material or corporeal substance.

Though all bodies or solid substances resist, yet all those with which we are acquainted appear to be more or less compressible, or capable of having, without any diminution in the quantity of their matter, their bulk more or less reduced within a smaller space than that which they usually occupy. An experiment of the Florentine academy was supposed to have fully demonstrated that water was absolutely incompressible. The same experiment, however, having been repeated with more care and accuracy, it appears, that water, though it strongly resists compression, is, however, when a sufficient force is applied, like all other bodies, in some degree liable to it. Air, on the contrary, by the application of a very moderate force, is easily reducible within a much smaller portion of space than that which it usually occupies. The condensing engine, and what is founded upon it, the wind-gun, sufficiently demonstrate this: and even without the help of such ingenious and expensive machines, we may easily satisfy ourselves of the truth of this proportion, by squeezing a full-blown bladder of which the neck is well tied.

The hardness or softness of bodies, or the greater or smaller force with which they resist any change of shape, seems to depend altogether upon the stronger or weaker degree of cohesion with which their parts are mutually attracted to one another. The greater or smaller force with which they resist compression may, upon many occasions, be owing partly to the same cause: but it may likewise be owing to the greater or smaller proportion of empty space comprehended within their dimensions, or intermixed with the solid parts which compose them. A body which comprehended no empty space within its dimensions, which, through all its parts, was completely filled with the resisting substance, we are naturally disposed to conceive as something which would be absolutely incompressible, and which would resist, with unconquerable force, every attempt to reduce it within narrower dimensions. If the solid and resisting substance, without moving out of its place, should admit into the same place another solid and resisting substance, it would from that moment, in our apprehension, cease to be a solid and resisting substance, and would no longer appear to possess that quality, by which alone it is made known to us, and which we therefore consider as constituting its nature and essence, and as altogether inseparable from it. Hence our notion of what has been called impenetrability of matter; or of the absolute impossibility that two 443 solid resisting substances should occupy the same place at the same time.

This doctrine, which is as old as Leucippus, Democritus, and Epicurus, was in the last century revived by Gassendi, and has since been adopted by Newton and the far greater part of his followers. It may at present be considered as the established system, or as the system that is most in fashion, and most approved of by the greater part of the philosophers of Europe. Though it has been opposed by several puzzling arguments, drawn from that species of metaphysics which confounds every thing and explains nothing, it seems upon the whole to be the most simple, the most distinct, and the most comprehensible account that has yet been given of the phenomena which are meant to be explained by it. I shall only observe, that whatever system may be adopted concerning the hardness or softness, the fluidity or solidity, the compressibility or incompressibility of the resisting substance, the certainty of our distinct sense and feeling of its Externality, or of its entire independency upon the organ which perceives it, or by which we perceive it, cannot in the smallest degree be affected by any such system. I shall not therefore attempt to give any further account of such systems.

Heat and cold being felt by almost every part of the human body, have commonly been ranked along with solidity and resistance, among the qualities which are the objects of Touch. It is not, however, I think, in our language proper to say that we touch, but that we feel the qualities of heat and cold. The word feeling, though in many cases we use it as synonymous to touching, has, however, a much more extensive signification, and is frequently employed to denote our internal, as well as our external, affections. We feel hunger and thirst, we feel joy and sorrow, we feel love and hatred.

Heat and cold, in reality, though they may frequently be perceived by the same parts of the human body, constitute an order of sensations altogether different from those which are the proper objects of Touch. They are naturally felt, not as pressing upon the organ, but as in the organ. What we feel while we stand in the sunshine during a hot, or in the shade during a frosty, day, is evidently felt, not as pressing upon the body, but as in the body. It does not necessarily suggest the presence of any external object, nor could we from thence alone infer the existence of any such object. It is a sensation which neither does nor can exist any where but either in the organ which feels it, or in the unknown principle of perception, whatever that may be, which feels in that organ, or by means of that organ. When we lay our hand upon a table, which is either heated or cooled a good deal beyond the actual temperature of our hand, we have two distinct perceptions: first, that of the solid or resisting table, which is necessarily felt as something external to, and independent of, the hand which feels it; and secondly, 444 that of the heat or cold, which by the contact of the table is excited in our hand, and which is naturally felt as nowhere but in our hand, or in the principle of perception which feels in our hand.

But though the sensations of heat and cold do not necessarily suggest the presence of any external object, we soon learn from experience that they are commonly excited by some such object: sometimes by the temperature of some external body immediately in contact with our own body, and sometimes by some body at either a moderate or a great distance from us; as by the fire in a chamber, or by the sun in a summer’s day. By the frequency and uniformity of this experience, by the custom and habit of thought which that frequency and uniformity necessarily occasion, the Internal Sensation, and the External Cause of that Sensation, come in our conception to be so strictly connected, that in our ordinary and careless way of thinking, we are apt to consider them as almost one and the same thing, and therefore denote them by one and the same word. The confusion, however, is in this case more in the word than in the thought; for in reality we still retain some notion of the distinction, though we do not always evolve it with that accuracy which a very slight degree of attention might enable us to do. When we move our hand, for example, along the surface of a very hot or of a very cold table, though we say that the table is hot or cold in every part of it, we never mean that, in any part of it, it feels the sensations either of heat or of cold, but that in every part of it, it possesses the power of exciting one or other of those sensations in our bodies. The philosophers who have taken so much pains to prove that there is no heat in the fire, meaning that the sensation or feeling of heat is not in the fire, have laboured to refute an opinion which the most ignorant of mankind never entertained. But the same word being, in common language, employed to signify both the sensation and the power of exciting that sensation, they, without knowing it perhaps, or intending it, have taken advantage of this ambiguity, and have triumphed in their own superiority, when by irresistible arguments they establish an opinion which, in words indeed, is diametrically opposite to the most obvious judgments of mankind, but which in reality is perfectly agreeable to those judgments.