answers to by far the deepest and most vital difference.  Mr Spencer’s is open to the radical objection, that it classifies truths not according to their subject-matter or their mutual relations, but according to an unimportant difference in the manner in which we come to know them.  Of what consequence is it that the law of inertia (considered as an exact truth) is not generalized from our direct perceptions, but inferred by combining with the movements which we see, those which we should see if it were not for the disturbing causes?  In either case we are equally certain that it is an exact truth:  for every dynamical law is perfectly fulfilled even when it seems to be counteracted.  There must, we should think, be many truths in physiology (for example) which are only known by a similar indirect process; and Mr Spencer would hardly detach these from the body of the science, and call them abstract and the remainder concrete.

[3] Systeme de Politique Positive, ii. 36.

[4] The strongest case which Mr Spencer produces of a scientifically ascertained law, which, though belonging to a later science, was necessary to the scientific formation of one occupying an earlier place in M. Comte’s series, is the law of the accelerating force of gravity; which M. Comte places in Physics, but without which the Newtonian theory of the celestial motions could not have been discovered, nor could even now be proved.  This fact, as is judiciously remarked by M. Littre, is not valid against the plan of M. Comte’s classification, but discloses a slight error in the detail.  M. Comte should not have placed the laws of terrestrial gravity under Physics.  They are part of the general theory of gravitation, and belong to astronomy.  Mr Spencer has hit one of the weak points in M. Comte’s scientific scale; weak however only because left unguarded.  Astronomy, the second of M. Comte’s abstract sciences, answers to his own definition of a concrete science.  M. Comte however was only wrong in overlooking a distinction.  There is an abstract science of astronomy, namely, the theory of gravitation, which would equally agree with and explain the facts of a totally different solar system from the one of which our earth forms a part.  The actual facts of our own system, the dimensions, distances, velocities, temperatures, physical constitution, &c., of the sun, earth, and planets, are properly the subject of a concrete science, similar to natural history; but the concrete is more inseparably united to the abstract science than in any other case, since the few celestial facts really accessible to us are nearly all required for discovering and proving the law of gravitation as an universal property of bodies, and have therefore an indispensable place in the abstract science as its fundamental data.