matters after the sixteenth century. Holding al "that a vacuum or space in which there is absclutely no body is repugnant to reason," and that an indivisible space-filling particle is self-contradictory, he was led to identify space and matter ; that is, to make matter as indispensable to space as space to matter. There is, then, but one kind of corporeal being, whose attribute is extension. and whose modes are motion and rest. The most famous application of the mechanical conceptions which he bases upon this first principle, is his theory of the planets, which are conceived to be embedded in a transparent medium, and to move with it, vortex fashion, about the sun.1

But the conception of the space-filling continuity of material substance owes its prominence at the present time to the experimental hypothesis of ether. This substance, originally conceived to occupy the intermolecular spaces and to serve as a medium for the propagation of undulations, is now regarded by many physicists as replacing matter. "It is the great hope of science at the present day," says a contemporary exponent of naturalism," that hard and heavy matter will be

Descartes distinguished his theory from that of Democritus in the Principles of Philosophy, Part IV, § ccii.

Such a theory

shown to be ether in motion." 5 would reduce bodies to the relative displacements of parts of a continuous substance, which would be first of all defined as spacial, and would possess such further properties as special scientific hypotheses might, require.

Two broadly contrasting theories thus appear: that which defines matter as a continuous substance coextensive with space; and that which defines it as a discrete substance divided by empty space. But both theories are seriously affected by the peculiarly significant development of the conception of force.

Motion and

§ 107. In the Cartesian system the cause of motion was pressure within a plenum. But in the seventeenth century this notion encountered the system of Newton, a system and Extension which seemed to involve action at a tion of Force. distance. In the year 1728 Voltaire

its Cause. Development

of the Concep

wrote from London:

"" 'When a Frenchman arrives in London, he finds a very great change, in philosophy as well as in most other things. In Paris he left the world all full of matter; here he finds absolute vacua. At Paris the universe is seen filled up with ethereal vortices, while here the same

'Pearson: Grammar of Science, pp. 259–260. Cf. ibid., Chap. VII, entire.

space is occupied with the play of the invisible forces c gravitation. In Paris the earth is painted for us longis like an egg, and in London it is oblate like a melon. A Paris the pressure of the moon causes the ebb and flo of tides; in England, on the other hand, the sea gravitate toward the moon, so that at the same time when the Parisians demand high water of the moon, the gentleme: of London require an ebb."

But these differences are not matters of taste, nor even rival hypotheses upon an equal footing. The Newtonian system of mechanics, the consummation of a development initiated by Galileo, differed from the vortex theory of Descartes as exact science differs from speculation and unverified conjecture. And this difference of method carried with it eventually certain profound differences of content, distinguishing the Newtonian theory even from that of Democritus, with which it had so much in common. Although Democritus had sought to avoid the element of purposiveness in the older hylozoism by referring the motions of bodies as far as possible to the impact of other bodies, he nevertheless attributed these motions ultimately to weight, signifying thereby a certain downward disposition. Now it is true that in his general belief Newton himself is not free from hylozoism. He thought of the motions of the

6 Quoted in Ueberweg: History of Philosophy, II, p. 124.

planets themselves as initiated and quickened by a power emanating ultimately from God. They are impressed by an intelligent Agent," and

66 can be the effect of nothing else than the wisdom and skill of a powerful ever-living Agent who, being in all places, is more able by his will to move the bodies within his boundless uniform sensorium, and thereby to form and reform the parts of the universe, than we are by our will to move the parts of our own bodies.""

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,

7 Quoted from the Opticks of Newton by James Ward, in his Naturalism and Agnosticism, I, p. 43.

reaction, and the like. The further explanatio of motion, the definition of a virtue or poten that produces it, first a neglected problem, then 2: irrelevant problem, is finally, for a naturalisti philosophy in which this progression is completed an insoluble problem. For the sequel to th purely descriptive procedure on the part of science is the disavowal of "metaphysics" by those wh will have no philosophy but science. Thus the scientific conservatism of Newton has led to the positivistic and agnostic phase of naturalism. Bur 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 de scribed 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

f