[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.

The ultimate cause of motion is God.  He has created bodies with an original measure of motion and rest, and, in accordance with his immutable character, he preserves this quantity of motion unchanged:  it remains constant in the world as a whole, though it varies in individual bodies.  For with the power to create or destroy motion bodies lack, further, the power to alter their quantity of motion.  By the side of God, the primary cause of motion, the laws of motion appear as secondary causes.  The first of these is the one become familiar under the name, law of inertia:  Everything continues of itself in the state (of motion or rest) in which it is, and changes its state only as a result of some extraneous cause.  The second of these laws, which are so valuable in mechanics, runs:  Every portion of matter tends to continue a motion which has been begun in the same direction, hence in a straight line, and changes its direction only under the influence of another body, as in the case of the circle above described.  Descartes bases these laws on the unchangeableness of God and the simplicity of his world-conserving (i.e., constantly creative) activity.  The third law relates to the communication of motion; but Descartes does not recognize the equality of action and reaction as universally as the fact demands.