(2) On the other hand, the _usefulness_ of the causal postulate depends entirely upon our ability to establish single threads of Causality within the stream of events, _i.e._ on our ability to assign particular assemblages of events, less than the “totality,” to particular subsequent events as their necessary and sufficient condition. Unless we can do this we can formulate no rules for the practical employment of means for the production of a desired result, and, as we have already seen, it is the necessity of knowing the means to our ends which is the primary, and indeed the sole, motive for the establishment of the causal postulate. Now, to effect this assignation of particular causes to particular effects, we have to make use of a distinction which is more practically necessary than theoretically defensible. We distinguish between indispensable conditions and accessory circumstances, which may or may not be present without affecting the nature of the special result in question. Now it is clear that the making of this distinction depends upon the separation of a certain part of the “total” stream of events from the rest, and its isolation as “the special result in question.” And this isolation, as we have seen, must always rest upon arbitrary abstraction. When once this arbitrary abstraction of some one part or aspect of the stream of events from its context has been made, we are compelled to recognise the existence of the context from which we have abstracted by saying that any effect may enter into or form part of a variety of different larger effects, according to the nature of the context in which it occurs. And, from the very principle of the complete correlation of condition and conditioned, it follows that what we call the special or partial effect will be preceded by varying conditions, according as it enters into different larger wholes or contexts. Thus any form of the causal postulate of which we can make effective use necessitates the recognition of that very Plurality of Causes which we have seen to be logically excluded by the conception of cause with which science works. As we contended above, any form of the principle in which it is true is useless, and any form in which it is useful is untrue.