While the principle of Causation, in its application to change of place on the part of bodies and their constituent atoms, is the leading thought in the De Motu, this essay also investigates articulately the nature of the phenomenon which we call motion (sect. 43-66). It assumes that motion is only an effect, seeing that no one who reflects can doubt that what is presented to our senses in the case of motion is altogether passive: there is nothing in the successive appearance of the same body in different places that involves action on the part of either of the moving or the moved body, or that can be more than inert effect (sect. 49). And all concrete motion, it is assumed, must be something that can be perceived by our senses. Accordingly it must be a perceptible relation between bodies, as far as it is bodily: it could make no appearance at all if space contained only one solitary body: a plurality of bodies is indispensable to its appearance. Absolute motion of a solitary body, in otherwise absolutely empty space, is an unmeaning abstraction, a collocation of empty words. This leads into an inquiry about relative space as well as relative place, and the intelligibility of absolute space, place, and motion (sect. 52-64).
Local motion is unintelligible unless we understand the meaning of space. Now some philosophers distinguish between absolute space, which with them is ultimately the only real space, and that which is conditioned by the senses, or relative. The former is said to be boundless, pervading and embracing the material world, but not itself presentable to our senses; the other is the space marked out or differentiated by bodies contained in it, and it is in this way exposed to our senses (sect. 52). What must remain after the annihilation of all bodies in the [pg 497] universe is relativeless, undifferentiated, absolute space, of which all attributes are denied, even its so-called extension being neither divisible nor measurable; necessarily imperceptible by sense, unimaginable, and unintelligible, in every way unrealisable in experience; so that the words employed about it denote nothing (sect. 53).
It follows that we must not speak of the real space which a body occupies as part of a space that is necessarily abstracted from all sentient experience; nor of real motion as change within absolute space, without any relation between bodies, either perceived or conceived. All change of place in one body must be relative to other bodies, among which the moving body is supposed to change its place—our own bodies which we animate being of course recognised among the number. Motion, it is argued, is unintelligible, as well as imperceptible and unimaginable, without some relation between the moving body and at least one other body: the truth of this is tested when we try to suppose the annihilation of all other bodies, our own included, and retain only a solitary globe: absolute motion is found unthinkable. So that, on the whole, to see what motion means we must rise above the mathematical postulates that are found convenient in mechanical science; we must beware of empty abstractions; we must treat motion as something that is real only so far as it is presented to our senses, and remain modestly satisfied with the perceived relations under which it then appears (sect. 65-66).
Finally, is motion, thus explained, something that can be spoken of as an entity communicable from one body to another body? May we think of it as a datum of sense existing in the striking body, and then passing from it into the struck body, the one losing exactly as much as the other receives? (sect. 67). Deeper thought finds in those questions only a revival of the previously [pg 498] exploded postulate of “force” as something sensible, yet distinct from all the significant appearances sense presents. The language used may perhaps be permitted in mathematical hypotheses, or postulates of mechanical science, in which we do not intend to go to the root of things. But the obvious fact is, that the moving body shews less perceptible motion, and the moved body more. To dispute whether the perceptible motion acquired is numerically the same with that lost leads into frivolous verbal controversy about Identity and Difference, the One and the Many, which it was Berkeley's aim to expel from science, and so to simplify its procedure and result. Whether we say that motion passes from the striking body into the struck, or that it is generated anew within the struck body and annihilated in the striking, we make virtually the same statement. In each way of expression the facts remain, that the one body presents perceptible increase of its motion and the other diminution. Mind or Spirit is the active cause of all that we then see. Yet in mechanical science—which explains things only physically, by shewing the significant connexion of events with their mechanical rules—terms which seem to imply the conveyance of motion out of one body into another may be pardoned, in consideration of the limits within which physical science is confined, and its narrower point of view. In physics we confine ourselves to the sensuous signs which arise in experience, and their natural interpretation, in all which mathematical hypotheses are found convenient; so that gravitation, for example, and other natural rules of procedure, are spoken of as causes of the events which conform to them, no account being taken of the Active Power that is ultimately responsible for the rules. For the Active Power in which we live, move, and have our being, is not a datum of sense; meditation brings it into light. But to pursue this thought would carry us beyond the physical laws of Motion (sect. 69-72).
The De Motu may be compared with what we found in the Principles, sect. 25-28 and 101-117. The total powerlessness of the significant appearances presented to the senses, and the omnipotence of Mind in the economy of external nature, is its chief philosophical lesson.
De Motu
1. Ad veritatem inveniendam præcipuum est cavisse ne voces males intellectæ[925] nobis officiant: quod omnes fere monent philosophi, pauci observant. Quanquam id quidem haud adeo difficile videtur, in rebus præsertim physicis tractandis, ubi locum habent sensus, experientia, et ratiocinium geometricum. Seposito igitur, quantum licet, omni præjudicio, tam a loquendi consuetudine quam a philosphorum auctoritate nato, ipsa rerum natura diligenter inspicienda. Neque enim cujusquam auctoritatem usque adeo valere oportet, ut verba ejus et voces in pretio sint, dummodo nihil clari et certi iis subesse comperiatur.