Now this assumption is even more evidently than the preceding a postulate. We have to make it, if we are to calculate the course of events, but we have no guarantee that it will succeed beyond the fact of its actual success. If it fails anywhere, as we shall hereafter contend that it does in the critical case of the sequence of psychical on physical events, and _vice versâ_, two results will follow, one practical, the other speculative. The practical consequence of the failure is that in such cases we cannot apply the concept of continuous process, and have to fall back upon the cruder notion of causal sequence. Thus, in attempting to create a science of Psychophysics we cannot hope to exhibit the whole of a psychophysical process as the continuous realisation of a single principle; we must be content to establish laws of causal connection between the physical and the psychical sides of the process. The speculative consequence is that the principle of Ground and Consequence is only imperfectly represented by the conception of a continuous process, inasmuch as that conception is only applicable where qualitative differences within the consequences of a single ground can be disregarded. This hint will prepare us for subsequent criticism of the concept of continuity when we come to deal with the metaphysics of the time-process.[110] #§ 8 (_b_). _The Indefinite Regress._ The defects of the causal postulate as a principle of explanation may also be exhibited by showing the double way in which it leads to the indefinite regress. The indefinite regress in the causal series is an inevitable consequence of the structure of time, and, as we please, may be detected both outside and inside any causal relation of two events, or two stages of a continuous process. For it follows from the structure of the time-series (_a_) that there are an indefinite number of terms of the series between any two members, between which there is a finite interval, and (_b_) that there is also an indefinite number of terms before or after any given member of the series. Like the series of real numbers, the time-series, because it satisfies the definition of a continuous infinite series, can have neither a first nor a last term, nor can any member of it have a next term.[111] Applying this to the case of Causation, we may reason as follows:—