But by the side of these statements must be set his famous disclaimer, "_hypotheses non fingo_." In his capacity of natural philosopher he did not seek to explain motions, but only to describe them. Disbelieving as he did in action at a distance, he saw no possibility of explanation short of a reference of them to God; but such "hypotheses" he thought to be no proper concern of science. As a consequence, the mathematical formulation of motions came, through him, to be regarded as the entire content of mechanics. The notion of an efficient cause of motion is still suggested by the term _force_, but even this term within the system of mechanics refers always to a definite amount of motion, or measurement of relative motion. And the same is true of _attraction_, _action_, _reaction_, and the like. The further explanation of motion, the definition of a virtue or potency that produces it, first a neglected problem, then an irrelevant problem, is finally, for a naturalistic philosophy in which this progression is completed, an insoluble problem. For the sequel to this purely descriptive procedure on the part of science is the disavowal of "metaphysics" by those who will have no philosophy but science. Thus the scientific conservatism of Newton has led to the positivistic and agnostic phase of naturalism. But a further treatment of this development must be reserved until the issue of epistemology shall have been definitely raised. A different emphasis within the general mechanical scheme, attaching especial importance to the conceptions of force and energy, has led to a rival tendency in science and a contrasting type of naturalism. The mechanical hypotheses hitherto described are all of a simple and readily depicted type. They suggest an imagery quite in accord with common-sense and with observation of the motions of great masses like the planets. Material particles are conceived to move within a containing space; the motions of corpuscles, atoms, or the minute parts of ether, differing only in degree from those of visible bodies. The whole physical universe may be represented in the imagination as an aggregate of bodies participating in motions of extraordinary complexity, but of one type. But now let the emphasis be placed upon the determining causes rather than upon the moving bodies themselves. In other words, let the bodies be regarded as attributive and the forces as substantive. The result is a radical alteration of the mechanical scheme and the transcendence of common-sense imagery. This was one direction of outgrowth from the work of Newton. His force of gravitation prevailed between bodies separated by spaces of great magnitude. Certain of the followers of Newton, notably Cotes, accepting the formulas of the master but neglecting his allusions to the agency of God, accepted the principle of action at a distance. _Force_, in short, _was conceived to pervade space of itself_.