If a body in motion meets another body, and its power (to continue its motion in a straight line) is less than the resistance of the other on which it has impinged, it retains its motion, but in a different direction:  it rebounds in the opposite direction.  If, on the contrary, its force is greater, it carries the other body along with it, and loses so much of its own motion as it imparts to the latter.  The seven further rules added to these contain much that is erroneous.  As actio in distans is rejected, all the phenomena of motion are traced back to pressure and impulse.  The distinction between fluid and solid bodies is based on the greater or less mobility of their parts.

The leading principle in the special part of the Cartesian physics,—­we can only briefly sketch it,—­which embraces, first, celestial, and, then, terrestial phenomena, is the axiom that we cannot estimate God’s power and goodness too highly, nor ourselves too meanly.  It is presumptuous to seek to comprehend the purposes of God in creation, to consider ourselves participants in his plans, to imagine that things exist simply for our sake—­there are many things which no man sees and which are of advantage to none.  Nothing is to be interpreted teleologically, but all must be interpreted from clearly known attributes, hence purely mechanically.  After treating of the distances of the various heavenly bodies, of the independent light of the sun and the fixed stars and the reflected light of the planets, among which the earth belongs, Descartes discusses the motion of the heavenly bodies.  In reference to the motion of the earth he seeks a middle course between the theories of Copernicus and Tycho Brahe.  He agrees with Copernicus in the main point, but, in reliance on his definition of motion, maintains that the earth is at rest, viz., in respect to its immediate surroundings.  It is clear that the harmony of his views with those of the Church (though it was only a verbal agreement) was not unwelcome to him.  According to his hypothesis,—­as he suggests, perhaps an erroneous hypothesis,—­the fluid matter which fills the heavenly spaces, and which may be compared to a vortex or whirlpool, circles about the sun and carries the planets along with it.  Thus the planets move in relation to the sun, but are at rest in relation to the adjacent portions of the matter of the heavens.  In view of the biblical doctrine, according to which the world and all that therein is was created at a stroke, he apologetically describes his attempt to explain the origin of the world from chaos under the laws of motion as a scientific fiction, intended merely to make the process more comprehensible.  It is more easily conceivable, if we think of the things in the world as though they had been gradually formed from elements, as the plant develops from the seed.  We now pass to the Cartesian anthropology, with its three chief objects:  the body, the soul, and the union of the two.