It may be said, however, that, quite apart from any belief in Divine agency, the existence of force as a cause of motion is commonly accepted by physicists. If we ask what is the cause of the attractions which, as a matter of observation, take place within a given sequence of configurations, we shall perhaps be told that it is the force of gravity. And if we require more exact information, it will be said that any two material particles exert upon each other an attractive force proportional to their mass, and varying inversely as the square of the distance between them. Is there, however, an observed antecedent force and then an observed sequent attraction? Surely not. From the physical point of view it is all one whether we speak of the force of gravitative attraction or the attraction of gravitative force. For physics, at any rate, according to its modern exponents, the attraction and the force are identical, save in so far as the technical term "force" is used to express a mathematical value which can be assigned to the observed strength of the attraction. We may cut out all reference to force in an exact statement of Newton's law without detracting from its scientific value, and say that the degree of the gravitative attraction of material particles is directly proportional to their mass and inversely as the square of their distance.
All that we know as to force and motion [wrote W. K. Clifford] is that a certain arrangement of surrounding bodies produces a certain alteration in the motion of a body. It has been usual to say that this arrangement of surrounding bodies produces a certain force, and that it is the action of this force that produces the alteration of the motion. Why have this intermediate term at all? Why should we not go at once from the surrounding circumstances to the alteration of motion which follows? The intermediate term is only a mental inference, either from the existence of the surrounding circumstances or from the occurrence of the alteration in the motion; and if we only accustom ourselves to pass from one to the other without its assistance it will cease to be necessary, and, like other useless mental conceptions, be gradually forgotten. And with it will pass all tendency to give to this useless mental phantom any such real and material qualities as indestructibility.
Science, therefore, deals exclusively with changes of configuration, and traces the accelerations which are observed to occur, leaving metaphysics to deal with questions concerning the underlying agency, if it exist.
Now in the case of an astronomical configuration we are dealing with accelerations of one order, those which are interpreted in terms of the ideal construction of gravitation. But in the physical universe there are many other orders of acceleration which have to be treated in terms of other formulae. The atom itself has recently been shown to be a complex configuration with most interesting electrical accelerations. There are configurations which must be dealt with under the laws of cohesion, of chemical affinity, of crystallisation, and so forth. And when we pass from one order of configuration to another, for example from that which is applicable to a solution of common salt undergoing evaporation to that which is applicable to the crystals which are presently formed, we find that different modes of acceleration emerge for treatment under new ideal constructions. We can indeed often formulate the conditions under which the new modes of acceleration are initiated; but that does not alter the fact that we are passing to different orders of configuration; it does, however, afford grounds for the belief in a natural relationship of any one order to another or others. When water-vapour condenses to the liquid state, and when the liquid freezes, we have in each case a new order of configuration which must be treated under the appropriate rules which experience has shown to be applicable. But they are related, and the conditions of the relationship can be discovered and discussed. New properties emerge; ice has properties which liquid water does not possess; but the conditions of their emergence can be formulated. Why they emerge science does not pretend to say. Such is the constitution of the nature we strive to interpret.
We are thus prepared to understand more fully the developed creed of naturalism. We have seen that, in the scientific interpretation of the motions of the planets, the antecedent configuration is termed the cause or condition of that which follows. Naturalism universalizes this conception. It regards the state of the whole universe at any given moment as a configuration of very great complexity, involving accelerations of many different orders coexisting in natural relationship, and it believes that the cause or condition of this configuration is that of the preceding moment, while the configuration of the succeeding moment is its effect. This involves a splendid act of faith, for it assuredly outruns what can, in the present state of knowledge, be definitely proved. It is the naturalistic creed of evolution. Beginning, so far as naturalism knows anything of beginnings, as a fire-mist or a swarm of meteoric particles, the solar system, with our earth as its most interesting constituent and man as its highest product, has reached its present condition. Again and again have new properties, new modes of acceleration, new types of interaction emerged, as minor configurations have been successively differentiated; but every such emergence has been rigidly conditioned and determined within the major configuration embracing the universe at large. In those cases where the conditions of emergence are as yet unknown, as, conspicuously, in the origin from not-living matter of the physical basis of life, with its characteristic properties and its puzzling physiological accelerations, we are bidden to believe, though we cannot establish by observation. This is part of the evolutionary creed for the earnest and consistent believer. I confess that as an evolutionist I am myself both ready and willing to believe; but I shall presently claim the right to exercise a like option in other fields of human thought, and in an interpretation of a different order. For the naturalistic creed deals only with the conditions of evolution. The conception of a causal agency of which evolution is the expression, if such indeed there be, is excluded from a naturalistic interpretation of nature so far as it is based on the methods of physical science.