Motion admits of no definition, from being but a local casuality of transitory endurance; motion is the same in all things, from an atom to a planet, against which all difference in velocity and direction makes no exception.

Impulsive pressure is the only cause analogous to the mechanical effect motion; pressure is universal because matter is inert.

Motion is not natural to inert matter: the term is expressive of the local condition of a body, while the body is prevented remaining where it is, and while the body is being passed through contiguous portions of space.

There is no cause of motion but physical impulse.

As effect and cause are necessarily equal, so is motion the measure of impulse, in time. Therefore as long as a body is in motion it is being impelled, however insensible the impelling cause. Motion must be in the direction of impulse; for, as a body cannot move itself, and is the passive patient of impulse, so must its direction be the same as that of impulse; therefore when the direction of motion is changed, it must be by a novel impulse in the novel direction.

From all matter being in motion, and all effect consisting in motion, and because like effects everywhere are attributable to the like or same cause, so must there be a cause of motion as universal as matter; rather than that there should be a distinct impelling cause for every individual motion following after the body, to put and keep it in motion. In all philosophic research the golden rule of nature should be held in mind, which prescribes "the shortest mode and fewest materials:" to mistake on the side of simplicity is more wise than censurable in the search after natural physical truths.

A universal cause of motion, it would seem, can be no other than a universal medium, a medium of pressure, one occupying the regions of planetary space, competent to keep the planets in interminable motion and effect all terrestrial minor motion: only by such means is it conceivable how the earth can be under endless, ever-varying impulse, productive of ever-changing direction. When impulsively pressed into motion by such a medium, the direction of a planet must be orbicular, on account of the pressure on the solar side being always less than on the opposite, by which the projectile direction is diverted from rectilinear to curvilinear.

Newton imagined that a medium, and however rare, occupying the regions of space, must retard, in time destroy, and eventually require the hand of Deity to restore the primeval order of planetary motion: no very bright idea of the great mathematician, considering the Omniscience of the Projector of a self-going, self-regulating Universe. Whereas a medium as dense as molten gold, could produce no such disorder as long as impulse is greater than resistance; which the long-continuance and order of planetary motion strongly seem to indicate is the case. Were there no medium in space, the planets must be at rest; one could not possibly affect another but by its shadow: Uranus being agitated by the greatly remote presence of Neptune, is proof of there being a connecting medium between. Gravitation is supposed to move the body possessed of the property, forwards,—why not every way?—to the sun or towards some neighbouring planet, but not to send that body or planet an agitating warning of its presence. How is gravitation within one planet to keep another in a state of agitation; which agitation being motion—a mechanical effect—is proof of there being a medium by which mediate connection is maintained between the two, Uranus and Neptune. Without a planetary medium there could be no system of planets. Suppose the existence of such a medium, then its sudden removal,—must not every subordinate system, which makes part of the universal system, become disjointed the same instant? Besides, from the laws of vision, rather of optics, there is equal proof that space contains a medium. There is no light to come from a star to the eye; there is nothing of sight belonging to the eye-balls; and there must be something between a star and the sense to connect the star with the sense; or how is the sense or brain to be so affected by the star, as that the perception or sensation shall be always the same when the eye-ball lenses are directed to the same star; and only by a universal medium can all the stars of the hemisphere be in connection with the eye at the same time, or the time of a few winks of the eye. Therefore until it is proved that constant planetary motion can be without constant and equal corresponding impulse, as to direction; and that a star can affect the sense of itself, immediately or with nothing between, all denial of planetary space being occupied by a medium of pressure, is utterly untenable.