That it is an ideal must not be forgotten. And that our most securely established generalisations are reached by carrying in thought to their conceptual limits the legitimate inferences from observations lacking in absolute universality and accuracy should be fully realised. Take the law of gravitation for example. It is sometimes asked by what right we assume from a limited number of observations very numerous perhaps but still limited that the law is universal; and, further, by what right we assume from measurements limited in accuracy very accurate, no doubt, but still falling short of that which is absolute—that in no particular case is there any variation, even by so much as a hair's breadth, from the formula which Newton expressed in mathematical terms. The answer is that we carry our law to an ideal limit unattainable by sense and by practical measurement. We assume that it is absolutely and universally true, because in no case has it been shown to be actually and observably false. We sweep our ideal curve through the recorded data of physical measurement, and regard the minute deviations of the actual from the ideal as due to errors of observation. We trust to a reality of thought which we believe to be truer and wider than the realities of sense.
Let us regard a rigidly naturalistic interpretation, then, as an ideal, partially and imperfectly realised to-day, and accepted in an attitude of belief which outruns the assured results of certain knowledge. That amounts to a faith in the universality of the determinate causal relationship as formulated by science. The first point to notice is that, as thus interpreted, facts and events are just simply and frankly accepted as given in experience. Why they are so given it is not the province of science to discuss. Let us realise quite clearly what this means. It is perhaps most readily grasped in terms of configuration—for example, in astronomy. At any given moment there are a number of material particles in motion at known velocities; to each particle is assigned a numerical coefficient which represents its mass. But the velocities are changing in amount or in direction or both, and such changes of velocities are termed accelerations. Now according to the first law of motion the velocity of any particle, if it could be isolated from the rest, would remain unchanged. There would be no acceleration. The accelerations imply the presence of other particles. Separate from the rest any two, A and B, in thought. The presence of A is the condition of the acceleration of B, and the presence of B of the acceleration of A. The term "force" is applied to the product obtained by multiplying, in this case, the mass of A by the acceleration of which the presence of B is a condition. It formulates in mathematical terms certain conditions within an ideal construction which can thus be so used as to enable us to predict the changes of velocity which will occur. It does not, for modern physics, assign a reason for these changes; in other words it has no reference to an agency by which the attraction may be produced. Force as a cause of acceleration is not for modern science a physical conception. Now when all the existing velocities and all the force-values within the configuration of any given moment are evaluated, the configuration of the succeeding moment can be predicted. Why the configuration should change in this particular way and not in some other way science does not attempt to explain; it suffices for astronomical physics to say that such is the constitution of nature, or, in other words such is the outcome of experience. The antecedent configuration is termed the cause or condition of that which follows. And this scientific use of the word "cause" should be carefully distinguished from the quite different use of the word by the theologian, who says that God has caused the planets to sweep round the sun in their appointed orbits. The conception of Divine or other agency does not fall within the physical universe of discourse.
If we accept, as a postulate of reason, the existence of some underlying cause, we must be careful to distinguish its agency from the antecedent conditions with which it is the province of science to deal. In the form in which the existence of a First Cause is often naively stated and humbly accepted we are bidden to trace a chain of scientific causation, in terms of antecedence and sequence, a long way back in time, and then when we have got as far back as ever we can, we are urged to posit just one more antecedent as the First Cause. But this antecedent is of a different nature from all the rest. They are configurations described in terms of matter and energy. This is no such configuration. Throughout the rest of the chain, wherever scientific explanation rules, any given configuration is not only the cause of the one which succeeds it, but the effect of some other that precedes it. But this is not true of the supposed First Cause.
Instead of an external First Cause, which in the beginning set the spheres a rolling along their appointed grooves, and stood aside, so to speak, while inorganic evolution ran its course, which then intervened to fashion protoplasm, and again stood aside till this new phase of evolutionary progress reached a certain stage, only to interfere once more, or more than once, to introduce new elements of consciousness and thought—in place of any such occasional influence of causal agency from without, we must rather accept that wider and more catholic belief in an immanent Power, omnipresent in space and time, of which the sequences of science are manifestations under the conditions of human experience. Nowise restricted to certain specific phenomena on certain specific occasions, its influence is seen here and now in the formation of a snow flake, as it was manifested at some stage in the evolutionary process in the terrestrial origin of protoplasm. But stress must again be laid on the fact that in any such conception the word "cause" is used in a different sense from that according to which it summarises the antecedent conditions with which it is the province of science to deal.