MOTION.

It is necessary to take notice of this term, because the idea which is named by it is apt to present the appearance of something mysterious, though, after the expositions with which we are now familiar, the materials of which it is compounded, will not be difficult to find.

The word Motion, is the abstract of Moving. What we have to investigate, therefore, are the sensations, on account of which, we call a body “moving;” motion being merely moving, the connotation dropped.

All motion is in a Line, either a straight line, or some other line. The idea of “moving,” therefore, contains, for one ingredient, the idea of a line.

A body “moving,” is a body which is successively at every point of a line. Every point of a line, as we have seen, is a particular position. A body “moving,” therefore, is a body first in one position, then in another, then in another, through a certain series.

In the idea of a Body moving, then, we can enumerate the following particulars: the idea of a body, the idea of a position, the idea of a line, the idea of succession. These are all complex ideas; some of them highly complex; united into one idea, motion, they compose one of the most complex of all our ideas. The ingredients, however, being already explained, there can be no great difficulty in understanding the compound.

It is commonly said, that motion includes the idea both of Space, and of Time. As it includes the idea of Succession, it includes the idea of Time, successions in the abstract (otherwise called instants), without end, receiving the name of Time. As it includes the idea of a Line, it includes the idea of extension in one direction. As it includes the idea of Position, which is that of lines, in every direction, it includes the idea of extension in every direction; but extension in every direction, taken abstractly, is Space.

It is important to observe, that, though we receive, and that the most frequently, information of motions by the eye, it is not from the sensations of sight, that the idea of motion is derived. It is by association of ideas alone, that we fancy we see motion, as it is thence we fancy that we see figure, and distance. The classes of sensations, from which we derive the idea of motion, and the idea of extension, are the same; they are the muscular and tactual sensations. The man born blind, is not without the idea of motion, as he is without that of colour; on the contrary, he has the idea probably much more precise, than we who have entangled it inextricably with the perceptions of sight.

To recur to the exposition which we have already given; we may remember, that the sensations (taking the simplest case), on account of which we apply the name Line, are partly sensations of Touch, partly sensations of Muscular Action. If we touch a line at one point with any part of our bodies, say the finger; so long as the finger is still, we have merely the sensations, on account of which we call the line tangible. As soon as we move the finger along the line, we have the sensations and ideas, on account of which we call it extended. But these new feelings, on account of which we call the line extended, are also the feelings, on account of which we call the finger moved. The sensations, therefore, whence we derive our ideas of extension, and of motion, exist simultaneously. We have a certain compound of feelings, partly sensations, and partly ideas; for we have already seen, that the perception of succession consists in a present sensation, associated with the idea of a past one; and we assign to this compound, not one name, as on other occasions, but two names, after a very peculiar and remarkable manner. These two names are, Line Extended, Finger Moved. The complication of the feelings here, and of course the obscurity of them, is very remarkable; though the naming, as in certain other cases of obscure ideas, is very distinct. We are never misled in the application of the terms, Line Extended, Finger moved; though we may be very much puzzled to shew, of the compound of feelings which are thus named, and which, in the compound, are easily, and infallibly traced, how much is included under the one term, and how much under the other. A certain portion of the sensations in the compound is peculiar to what is called the Line, another portion is peculiar to what is called the Finger. The rest is common to both. The common part, united to what is peculiar to line, is called line extended; the same common part, united with what is peculiar to finger, is called Finger moved.

Our ideas of extension and motion, are, no doubt, originally derived from the action of our own bodies. I touch something, and have the sensation of resistance. The idea of resistance is the fundamental part in every combination to which I give the name of object. In this case, there is the object touched, and there is the finger touching. A certain action is given to my finger, still touching the object. That action involves certain feelings; these I combine both with the object, and with the finger, and to these two combinations I give the two names, Object Extended, Finger Moved.

If any one shut his eyes, excluding as much as possible, the ideas of sight, and conceiving, without admixture, the feelings in the finger and the arm, while the finger passes along a line, he will get some notion of the series of antecedents and consequents, whence the idea of Motion is derived. They are feelings, which language does not enable us to communicate by words; but it does not seem very difficult for any man to raise the ideas of them in himself.

Let any one suppose, that the line commences opposite to the centre of his body. He begins by touching it at that point with the finger of his right hand; and in this there is one state of feeling. He gives the finger the smallest perceptible motion towards the right: this is another state of feeling. He gives it a further motion, the smallest perceptible, in the same direction: this is another state of feeling; and so on, as far as the arm can reach. The antecedent states are in each instance united with the present by memory, and by the amount of the states, thus united, the amount of the motion is computed.

Conceiving the case of a man born blind, the more easily to exclude the illusions of association; it is obvious, that such a man can obtain the idea of another body in motion, only by accompanying it with his hand; or by associating the ideas, on account of which he calls the hand, moved, with the body in question. By frequent operations of the hand, such as that described above, he becomes familiar with the idea of the hand moved. The ideas of the sensations, on account of which, he calls it moved, are easily raised, easily form themselves into combination, and easily associate themselves with the object, Hand. The idea of Hand, and the idea of Hand moved, having become very familiar, it is an easy case of association to transfer the term moved to other things, as the foot moved, the body moved, the stone moved. When he has become familiar with the application of Moved, as a connotative term, to various objects, it is easy, in this, as in other cases, to drop the connotation; and then he has the abstract, MOTION.31 32