ask, why is this difference between the two bodies? An answer to this question can only then be regarded as satisfactory from the episteomological standpoint when the thing adduced as the cause is an observable fact of experience. The law of causality has the sense of a definite statement about the world of experience only when observable facts alone appear as causes and effects.
The Newtonian mechanics does not give to this question any satisfactory answer. For example, it says:—The laws of mechanics hold true for a space R_{1} relative to which the body S_{1} is at rest, not however for a space relative to which S_{2} is at rest.
The Galiliean space, which is here introduced is however only a purely imaginary cause, not an observable thing. It is thus clear that the Newtonian mechanics does not, in the case treated here, actually fulfil the requirements of causality, but produces on the mind a fictitious complacency, in that it makes responsible a wholly imaginary cause R_{1} for the different behaviours of the bodies S_{1} and S_{2} which are actually observable.
A satisfactory explanation to the question put forward above can only be thus given:—that the physical system composed of S_{1} and S_{2} shows for itself alone no conceivable cause to which the different behaviour of S_{1} and S_{2} can be attributed. The cause must thus lie outside the system. We are therefore led to the conception that the general laws of motion which determine specially the forms of S_{1} and S_{2} must be of such a kind, that the mechanical behaviour of S_{1} and S_{2} must be essentially conditioned by the distant masses, which we had not brought into the system considered. These distant masses, (and their relative motion as regards the bodies under consideration) are then to be looked upon as the seat of the principal observable causes for the different behaviours
