The objects of Touch always present themselves as pressing upon, or as resisting the particular part of the body which perceives them, or by which we perceive them. When I lay my hand upon the table, the table presses upon my hand, or resists the further motion of my hand, in the same manner as my hand presses upon the table. But pressure or resistance necessarily supposes externality in the thing which presses or resists. The table could not press upon, or resist the further motion of my hand, if it was not external to my hand. I feel it accordingly as something which is not merely an affection of the hand, but altogether external to and independent of my hand. The agreeable, indifferent, or painful sensation of pressure, accordingly as I happen to press hardly or softly, I feel, no doubt, as affections of my hand; but the thing which presses and which resists I feel as something altogether different from those affections, as external to my hand, and as being altogether independent of it.
In moving my hand along the table it soon comes, in every direction, to a place where this pressure or resistance ceases. This place we call the boundary, or end of the table; of which the extent and figure are determined by the extent and direction of the lines or surfaces which constitute this boundary or end.
It is in this manner that a man born blind, or who has lost his sight so early that he has no remembrance of visible objects, may form the most distinct idea of the extent and figure of all the different parts of his own body, and of every other tangible object which he has an opportunity of handling and examining. When he lays his hand upon his foot, as his hand feels the pressure or resistance of his foot, so his foot feels that of his hand. They are both external to one another, but they are, neither of them, altogether so external to him. He feels in both, and he naturally considers them as parts of himself, or at least as something which belongs to him, and which, for his own comfort, it is necessary that he should take some care of.
When he lays his hand upon the table, though his hand feels the pressure of the table, the table does not feel, at least he does not know that it feels, the pressure of his hand. He feels it therefore as something external, not only to his hand, but to himself, as something which makes no part of himself, and in the state and condition of which he has not necessarily any concern.
440 When he lays his hand upon the body either of another man, or of any other animal, though he knows, or at least may know, that they feel the pressure of his hand as much as he feels that of their body: yet as this feeling is altogether external to him, he frequently gives no attention to it, and at no time takes any further concern in it than he is obliged to do by that fellow-feeling which Nature has, for the wisest purposes, implanted in man, not only towards all other men, but (though no doubt in a much weaker degree) towards all other animals. Having destined him to be the governing animal in this world, it seems to have been her benevolent intention to inspire him with some degree of respect, even for the meanest and weakest of his subjects.
This power or quality of resistance we call Solidity; and the thing which possesses it, the Solid Body or Thing. As we feel it as something altogether external to us, so we necessarily conceive it as something altogether independent of us. We consider it, therefore, as what we call a Substance, or as a thing that subsists by itself, and independent of any other thing. Solid and substantial, accordingly, are two words which, in common language, are considered either as altogether or as nearly synonymous.
Solidity necessarily supposes some degree of extension, and that in all the three directions of length, breadth, and thickness. All the solid bodies, of which we have any experience, have some degree of such bulk or magnitude. It seems to be essential to their nature, and without it, we cannot even conceive how they should be capable of pressure or resistance; are are the powers by which they are made known to us, and by which alone they are capable of acting upon our own, and upon all other bodies.
Extension, at least any sensible extension, supposes divisibility. The body may be so hard, that our strength is not sufficient to break it; we still suppose, however, that if a sufficient force were applied, it might be so broken; and, at any rate, we can always, in fancy at least, imagine it to be divided into two or more parts.
Every solid and extended body, if it be not infinite, (as the universe may be conceived to be,) must have some shape or figure, or be bounded by certain lines and surfaces.