of events, and, within limits, to guide the latter to the fulfilment of our ends. The realization of this ability is the basis of the notion we form of efficiency, and in it the root of the concept of causality is grounded. The concrete meaning of causality for us is therefore the efficient determination of one thing by another. This relation of efficient determination is one-directioned ; it is not reciprocal. This follows from the fact of the one-directionality of time in actual experience. No doubt if we formulate symbolically this sequence of cause and effect, there is logical dependence of one on the other. Such a logical dependence, however, is descriptive, and does not alter the fact that in actual experience our activity determines its consequences in an entirely different sense from that in which it is determined by them.
In the course of his development, man comes to apply this idea of efficient causality to the sequences which he observes in the object of experience. The question then arises as to whether this application is valid. If we take the sequences simply as they stand, it is certainly not valid. We can only say that certain sequences do occur, and that we are able to formulate propositions in virtue of which the occurrence of some events can be inferred from the occurrence of others. From this purely scientific standpoint, causality is merely a logical and descriptive dependence of one event upon another. We may, however, wish to go beyond the mere existence of the sequences in an endeavor to find some satisfactory explanation of their existence as sequences. We know that some efficient individuals exist, and we also know that some of the sequences observed are initiated by the activity of these individuals. Hence we take as an hypothesis the proposition that all sequences have their ground in the activity of efficient individuals. This hypothesis is not logically proven, but it covers the facts by explaining them in terms of entities whose nature we can realize. It is therefore justifiable.
It is necessary for the purposes of reasoning to formulate our ideas in terms of sequences of sense-data. But the artificiality of the latter is brought out by the consideration of the problem of continuity. In fact, the problem itself is due to this artifi-
