just the law of causality, which is a form of the principle of sufficient reason, and therefore cannot be further explained, because the principle of sufficient reason is the principle of all explanation and of all necessity. From this it is clear that cause and effect stand in intimate connection with, and necessary relation to, the _course of time_. Only because the state A. precedes in time the state B., and their succession is necessary and not accidental, _i.e._, no mere sequence but a consequence—only because of this is the state A. cause and the state B. effect. The conception _reciprocity_, however, contains this, that both are cause and both are effect of each other; but this really amounts to saying that each of the two is the earlier and also the later; thus it is an absurdity. For that both states are simultaneous, and indeed necessarily simultaneous, cannot be admitted, because, as necessarily belonging to each other and existing at the same time, they constitute only _one_ state. For the permanence of this state there is certainly required the continued existence of all its determinations, but we are then no longer concerned with change and causality, but with duration and rest, and nothing further is said than that if _one_ determination of the whole state be changed, the new state which then appears cannot continue, but becomes the cause of the change of all the other determinations of the first state, so that a new third state appears; which all happens merely in accordance with the simple law of causality, and does not establish a _new_ law, that of reciprocity.