In a communication to the Philosophical Magazine, in 1867, by James Croll, of Edinburgh, "On certain Hypothetical Elements in the Theory of Gravitation," the author revived the difficulties which had been felt and proclaimed by Faraday. He says : " It was demonstrated by Newton, and has been proved by general observation and experience, that [262] bodies tend toward each other with a force varying inversely as the square of the distance, and directly as the mass of the bodies. But it never was demonstrated or proved by any one that the bodies attract each other. The thing which has been demonstrated is that B tends toward A; but the theory does not rest here ; it goes on to account for this tendency by referring it to a hypothetical cause, viz, to the ' attraction' of A. This however is a mere hypothesis, and no way essential to the theory. All that the theory requires is that it be demonstrated that A tends to move toward B. It is not necessary that we should go beyond this, and attempt to explain the cause of this tendency. Trifling as this assumption included in the theory may at first sight appear, it will be found that almost all the difficulties and objections which have been urged against the theory of gravitation are due in some form or other to that assumption. At the very outset we have the objection urged against the theory that it implies the absurdity of action at a distance. Now the mere facts of gravitation imply no such thing. That A and B placed at a distance should tend toward each other does not imply action at a distance. A moves by virtue of a force, but it does not follow that this force is at a distance from A. But if we assert that A and B 'attract' each other, then we imply action at a distance; for A is then affirmed to move in consequence of the force of B, and B in consequence of the force of A. ' The very idea of attractive force,' as Professor Brücke remarks, ' includes that of an action at a distance.' No principle will ever be generally received that stands in opposition to the old adage, 'A thing cannot act where it is not,' any more than it would were it to stand in opposition to that other adage, 'A thing cannot act before it is, or when it is not.'"
These venerable "adages" are about as valuable in directing us to the actual facts of nature, as that other celebrated adage of Zeno, " a body cannot move where it is not;" and conversely, "it cannot move where it is." An equally profound dictum is, that a " cause " cannot properly be said to precede its " effect," since succession implies discontinuity.
It may be a fact of natural law that everything acts where it is not," including even an aetherial vibration ; and certainly there is no difficulty in believing it; and the other metaphysical axiom maybe easily discredited by the simple reflection, that were our sun suddenly blotted from existence by supreme power, though all " attraction" of the planets would instantly cease, its full dynamic action on the earth would continue unimpaired for eight minutes. Were Sirius annihilated this year, it would still continue to pour upon us its lull measure of dynamic action for twenty years " when it was not." The difficulty is not in the possibility of posthumous action, but in the possibility of annihilation.
As Mill has very properly stated in answer to Sir William Hamilton, "Action at a distance is intrinsically quite as credible as action in [263] contact, and there is no reason, apart from specific experience, to regard the one as in any respect less probable than the other."
In our profound ignorance of all beyond the ascertained facts of gravitation, there could be no objection whatever to the substitution of the word "tend" for the word "attract," did it adequately express the observed fact that A induces in B a " tendency" to approach ; the quantity of tendency in B being found to be proportional not merely to its own mass, but notably to the mass of the distant body A.