individual, determinate, limited extension. In reality, wherever extension is, there substance is also,--the non-existent has no extension,--and wherever space is, there matter is also. Empty space does not exist. When we say a vessel is empty, we mean that the bodies which fill it are imperceptible; if it were absolutely empty its sides would touch. Descartes argues against the atomic theory and against the finitude of the world, as he argues against empty space: matter, as well as space, has no smallest, indivisible parts, and the extension of the world has no end. In the identification of space and matter the former receives fullness from the latter, and the latter unlimitedness from the former, both internal unlimitedness (endless divisibility) and external (boundlessness). Hence there are not several matters but only one (homogeneous) matter, and only one (illimitable) world. [Footnote 1: They are merely subjective states in the perceiver, and entirely unlike the motions which give rise to them, although there is a certain agreement, as the differences and variations in sensation are paralleled by those in the object.] Matter is divisible, figurable, movable quantity. Natural science needs no other principles than these indisputably true conceptions, by which all natural phenomena may be explained, and must employ no others. The most important is motion, on which all the diversity of forms depends. Corporeal being has been shown to be extension; corporeal becoming is motion. Motion is defined as "the transporting of one part of matter, or of one body, from the vicinity of those bodies that are in immediate contact with it, or which we regard as at rest, to the vicinity of other bodies." This separation of bodies is reciprocal, hence it is a matter of choice which shall be considered at rest. Besides its own proper motion in reference to the bodies in its immediate vicinity, a body can participate in very many other motions: the traveler walking back and forth on the deck of a ship, for instance, in the motion of the vessel, of the waves, and of the earth. The common view of motion as an activity is erroneous; since it requires force not only to set in motion bodies which are at rest, but also to stop those which are in motion, it is clear that motion implies no more activity than rest. Both are simply different states of matter. Since there is no empty space, each motion spreads to a whole circle of bodies: A forces B out of its place, B drives out C, and so on, until Z takes up the position which A has left.