Once, and once only, did Laplace launch forward, like Kepler, like Descartes, like Leibnitz, like Buffon, into the region of conjectures.  But then his conception was nothing less than a complete cosmogony.  All the planets revolve around the sun, from west to east, and in planes only slightly inclined to each other.  The satellites revolve around their respective primaries in the same direction.  Both planets and satellites, having a rotary motion, turn also upon their axes from west to east.  Finally, the rotation of the sun also is directed from west to east.  Here, then, is an assemblage of forty-three movements, all operating alike.  By the calculus of probabilities, the odds are four thousand millions to one that this coincidence in direction is not the effect of accident.

It was Buffon, I think, who first attempted to explain this singular feature of our solar system.  “Wishing, in the explanation of phenomena, to avoid recourse to causes which are not to be found in nature,” the celebrated academician sought for a physical cause for what is common to the movements of so many bodies differing as they do in magnitude, in form, and in their distances from the centre of attraction.  He imagined that he had discovered such a physical cause by making this triple supposition:  a comet fell obliquely upon the sun; it pushed before it a torrent of fluid matter; this substance, transported to a greater or less distance from the sun according to its density, formed by condensation all the known planets.  The bold hypothesis is subject to insurmountable difficulties.  I proceed to indicate, in a few words, the cosmogonic system which Laplace substituted for it.

According to Laplace, the sun was, at a remote epoch, the central nucleus of an immense nebula, which possessed a very high temperature, and extended far beyond the region in which Uranus now revolves.  No planet was then in existence.  The solar nebula was endowed with a general movement of rotation in the direction west to east.  As it cooled it could not fail to experience a gradual condensation, and in consequence to rotate with greater and greater rapidity.  If the nebulous matter extended originally in the plane of its equator, as far as the limit where the centrifugal force exactly counterbalanced the attraction of the nucleus, the molecules situate at this limit ought, during the process of condensation, to separate from the rest of the atmospheric matter and to form an equatorial zone, a ring, revolving separately and with its primitive velocity.  We may conceive that analogous separations were effected in the remoter strata of the nebula at different epochs and at different distances from the nucleus, and that they gave rise to a succession of distinct rings, all lying in nearly the same plane, and all endowed with different velocities.