66. I cannot imagine how any philosopher so learned as Comte should not perceive the reduction of the phenomena of the universe to “different aspects” of the one faculty of gravitation to be utterly impossible. In the first place, it has been shown that gravitation could not be the basis of vis inertiæ, without which it cannot exist; and in the next place, gravitation has always, at any given point of time, its possible influence limited to the power of making a body move toward an appropriate centre of gravity, and afterward remain forever at rest, unless affected by some extraneous cause. 67. It is alleged also that the phenomena of the universe are explained by gravitation. I here quote his own words: 68. “_Our business is—seeing how vain is any research into what are called causes, whether first or final—to pursue an accurate discovery of these laws, with a view to reducing them to the smallest possible number._” 69. How is it possible, I demand, to reduce the orbitual motion of a planet to fewer causes than vis inertiæ, motion, and gravitation? Vis inertiæ and motion are necessary to momentum; and momentum thus arising, acting in a tangential direction to that of gravitation, is indispensable to form, with the force of gravity, the resultant which constitutes the orbitual curve. 70. Yet from subsequent language in the same paragraph, the idea is suggested of reducing planetary motions to one cause, gravitation! This will be perceived from his language, subjoined as follows: 71. “_The best illustration of this is in the case of the doctrine of gravitation. We say that the general phenomena of the universe are explained by it, because it connects, under one head, the whole immense variety of astronomical facts, exhibiting the constant tendency of atoms toward each other in direct proportion to their masses, and in inverse proportion to the squares of their distance._” 72. How can the revolution of a single planet about the sun be explained without the centrifugal or tangential force due to momentum? Were not gravitation resisted by the projectile velocity constituted by motion and vis inertiæ, would not all the planets fall into their suns, respectively? 73. Are there not three essential elements in such orbitual movements,—vis inertiæ, motion, and gravitation? Are not these as necessary to an orbit as three sides are to a triangle? and is it not as great an error to suppose that such movements can continue by the agency of _one_ of them, as to make one right line serve to enclose a superficies?