I also definitely assert that the conception _reciprocity_ cannot be supported by a single example. Everything that one seeks to pass off as such is either a state of rest, to which the conception of causality, which has only significance with reference to changes, finds no application at all, or else it is an alternating succession of states of the same name which condition each other, for the explanation of which simple causality is quite sufficient. An example of the first class is afforded by a pair of scales brought to rest by equal weights. Here there is no effect produced, for there is no change; it is a state of rest; gravity acts, equally divided, as in every body which is supported at its centre of gravity, but it cannot show its force by any effect. That the taking away of one weight produces a second state, which at once becomes the cause of the third, the sinking of the other scale, happens according to the simple law of cause and effect, and requires no special category of the understanding, and not even a special name. An example of the second class is the continuous burning of a fire. The combination of oxygen with the combustible body is the cause of heat, and heat, again, is the cause of the renewed occurrence of the chemical combination. But this is nothing more than a chain of causes and effects, the links of which have alternately _the same name_. The burning, A., produces free heat, B., this produces new burning, C. (_i.e._, a new effect which has the same name as the cause A., but is not individually identical with it), this produces new heat, D. (which is not really identical with the effect B., but only according to the concept, _i.e._, it has the same name), and so on indefinitely. A good example of what in ordinary life is called reciprocity is afforded by a theory about deserts given by Humboldt (_Ansichten der Natur_, 2d ed., vol. ii. p. 79). In the sandy deserts it does not rain, but it rains upon the wooded mountains surrounding them. The cause is not the attraction of the clouds by the mountains; but it is the column of heated air rising from the sandy plain which prevents the particles of vapour from condensing, and drives the clouds high into the heavens. On the mountains the perpendicular rising stream of air is weaker, the clouds descend, and the rainfall ensues in the cooler air. Thus, want of rain and the absence of plants in the desert stand in the relation of reciprocity; it does not rain because the heated sand-plain sends out more heat; the desert does not become a steppe or prairie because it does not rain. But clearly we have here again, as in the example given above, only a succession of causes and effects of the same names, and throughout nothing essentially different from simple causality. This is also the case with the swinging of the pendulum, and indeed also with the self-conservation of the organised body, in which case likewise