From this point of view the one-sided dependence of effect on cause, characteristic of the causal relation, disappears. Whether we shall infer the later stages of the process from the earlier, or the earlier from the later, depends simply upon our choice of positive or negative values for our time-variable. For “descriptive” science, what we suggested at first as a paradoxical possibility is the actual fact. The past is determined by the future in precisely the same sense in which the future is determined by the past, namely, that as both are stages of the same continuous process, if once you know the principle of the process you can start equally well with either and reason to the other.[109] Thus, within the limits of experimental science itself, the conception of causal relation has given way to the conception of events as logically connected into a system in virtue of their underlying ground or principle. For practical purposes experimental science has, in its application of this conception, to be guided by two postulates, neither of which can be metaphysically justified. It has to assume (_a_) that the course of events is composed of a plurality of more or less independent continuous processes, each of which has its own ground within itself, at least to such an extent as to be capable of being treated for our purpose as independent of others; (_b_) that the underlying ground or grounds of all events can be adequately expressed in terms of mathematical symbolism. As to the first of these points, our discussion of the unity of Reality convinced us that there must in the end be a single ground of all existence, and therefore the complete reason of any partial process cannot be entirely within itself. The independence of the various processes must be relative, and even the belief that it is sufficient to enable us to treat them for our own special purposes as self-contained and independent, must be a postulate prompted by our practical needs, and justified in the end by its success. The second point will engage our attention more fully in subsequent chapters. At present one remark upon it must suffice. The calculability of the laws of continuous processes depends upon our ability to reduce them to numerical and quantitative forms. Wherever we have the appearance, at any stage in a process, of a new quality, we have in fact an apparent breach of continuity, and it ceases to be in our power to exhibit the new stage of the process as a mere transformation of what was already expressed in former stages. Hence the success of natural science in reducing all sequences of events to continuous processes depends upon the assumption that we can establish equations between qualitatively different magnitudes.