The answer, succinctly put, is this: explaining or understanding things means tracing effects to their causes, and this is only one mode, a somewhat _disguised_ mode, of the joint functional activity of which I have spoken. The manifold in this case is that of the temporal sequence of phenomena, of differences due to change of position in time; and the unity established between them (as for instance energy, of which the sequent phenomena represent the transformations) is an ideal, fictive unity, mentally superimposed (real despite its ideal or imaginary character, because of the necessity we are under to view the sequent phenomena _sub specie unitatis_). That there is nothing in the antecedent to compel the sequent to follow has been since the days of Hume a commonplace in philosophy. That nevertheless there is such a thing as the prediction of eclipses was made by Kant the basis of his doctrine of synthesis _a priori_. Be the terms used what they may, what counts is the fact that the _joint action of two functions, which itself is inexplicable_, not to be understood, that is, not to be referred back to a preceding cause (as if there could be such a thing as a cause why we think in terms of causality) is the foundation of all so-called understanding. Moreover causality is an incomplete example of the fundamental functional process. We never do thoroughly understand; we gain a certain relief, a certain increased ease of mind by pushing the problem back a step. And what I have called the prejudice of causality, is the unwillingness on our part to acknowledge the fact that we are face to face, in the case of causality, with the inexplicable; that that which helps us partially to understand (and serves for practical purposes well enough) is in its nature not to be understood, one of the modes in which the joint action of the functional finalities manifests itself.