In speaking of figures, as employed by Nature, I do not mean to imply that they must be necessarily, or are exactly, similar to those geometrical figures which we form in our imagination. We form laws by supposition, and then endeavour to render them simple by abstraction. It is very possible that there are neither exact cubes nor perfect spheres in the universe; but as nothing certainly exists without form, and as from the variation of substances the figures of the elements are different, some of them most undoubtedly must approach to the sphere, the cube, and all the other regular figures which we have conceived. The precise, absolute, and abstract figures which our minds are so frequently induced to admit, cannot have any existence, because all objects are related, and differ only by almost imperceptible shades. It is by the same rule that when I speak of one substance as being entirely full, because composed of cubical particles, and another as being not more than half full, because its parts are spherical, I mean only comparatively, and not that such substances really exist; for experience has fully informed us that in transparent bodies, such as glass, which is both dense and heavy, there is but a small quantity of matter in proportion to the extent of the intervals; nay, as we have before observed, it might be demonstrated that even gold, which is the most dense species of matter, has more vacuities than substance. To investigate the powers of Nature is the object of rational mechanics, while active mechanics is solely confined to a combination of particular powers, and consequently the art of constructing machines. This art has at all times been certain of cultivation from necessity and convenience; and both ancients and moderns have equally excelled in it. But rational mechanics is a science invented in our days; for, from the days of Aristotle to those of Descartes, even the philosophers have reasoned no better upon the nature of motion, than uniformly to mistake the effect for the cause. Impulsion was the only force with which they were acquainted; to it they attributed the effects of others, and all the phenomena of the universe. If this idea of theirs had been probable, or even possible, impulsion, which they regarded as the sole cause, must have been a general effect, which equally belonged to all matter, and which equally exerted itself in all places, and at all times; but every day demonstrated the contrary to be the fact; for they must have perceived that this force had no existence in bodies at rest; that it had but a short subsistence in projected bodies, being soon destroyed by resistance; that a fresh impulse was absolutely necessary for its renewal, and that, consequently, so far from being a general cause, it was only a particular effect produced by others more general.