“How is the mutual action of atoms existing by themselves in complete insulation, and wholly without contact, to be realized in thought? We are here in presence of the old difficulties respecting the possibility of _actio in distans_ which presented themselves to the minds of the physicists in Newton’s time, and constituted one of the topics of the famous discussion between Leibnitz and Clarke, in the course of which Clarke made the remarkable admission that ‘if one body attracted another without an intervening body, that would be not a miracle, but a contradiction; for it would be to suppose that a body acts where it is not’—otherwise expressed: Inasmuch as action is but a mode of being, the assertion that a body can act where it is not would be tantamount to the assertion that a body can be where it is not. This admission was entirely in consonance with Newton’s own opinion; indeed, Clarke’s words are but a paraphrase of the celebrated passage in one of Newton’s letters to Bentley, cited by John Stuart Mill in his _System of Logic_, which runs as follows: ‘It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter _without mutual contact_.... That gravity should be innate, inherent, and essential to matter, so that one body may act on another, at a distance, through a vacuum, without the mediation of anything else by and through which their action and force be conveyed from one to the other, is to me so great an absurdity that I believe no man, who in philosophical matters has a competent faculty of thinking, can ever fall into it.’ ”