"There is in this theory," Mr Mill proceeds, "no unknown substance introduced upon supposition, nor any unknown property or law ascribed to a known substance. The known laws of matter authorize us to suppose, that a body which is constantly giving out so large an amount of heat as the sun is, must be progressively cooling, and that by the process of cooling it must contract; if, therefore, we endeavour, from the present state of that luminary, to infer its state in a time long past, we must necessarily suppose that its atmosphere extended much further than at present, and we are entitled to suppose that it extended as far as we can trace those effects which it would naturally leave behind it on retiring; and such the planets are. These suppositions being made, it follows from known laws that successive zones of the solar atmosphere would be abandoned; that these would continue to revolve round the sun with the same velocity as when they formed part of his substance, and that they would cool down, long before the sun himself, to any given temperature, and consequently to that at which the greater part of the vaporous matter of which they consisted would become liquid or solid. The known law of gravitation would then cause them to agglomerate in masses, which would assume the shape our planets actually exhibit; would acquire, each round its own axis, a rotatory movement; and would in that state revolve, as the planets actually do, about the sun, in the same direction with the sun's rotation, but with less velocity, and each of them in the same periodic time which the sun's rotation occupied when his atmosphere extended to that point; and this also M. Comte has, by the necessary calculations, ascertained to be true, within certain small limits of error. There is thus in Laplace's theory nothing hypothetical; it is an example of legitimate reasoning from a present effect to its past cause, according to the known laws of that case; it assumes nothing more than that objects which really exist, obey the laws which are known to be obeyed by all terrestrial objects resembling them."--Vol. II. p. 27.