Suppose nothing else were “given” as real except our world of desires and passions, and we could not get down, or up, to any other “reality” besides the reality of our drives–for thinking is merely a relation of these drives to each other: is it not permitted to make the experiment and to ask the question whether this “given” would not be sufficient for also understanding on the basis of this kind of thing the so-called mechanistic (or “material”) world?… In the end not only is it permitted to make this experiment; the conscience of method demands it. Not to assume several kinds of causality until the experiment of making do with a single one has been pushed to its utmost limit (to the point of nonsense, if I may say so)… The question is in the end whether we really recognize the will as efficient, whether we believe in the causality of the will: if we do–and at bottom our faith in this is nothing less than our faith in causality itself–then we have to make the experiment of positing causality of the will hypothetically as the only one. “Will,” of course, can affect only “will”–and not “matter” (not “nerves,” for example). In short, one has to risk the hypothesis whether will does not affect will wherever “effects” are recognized–and whether all mechanical occurrences are not, insofar as a force is active in them, will force, effects of will. Suppose, finally, we succeeded in explaining our entire instinctive life as the development and ramification of one basic form of the will–namely, of the will to power, as my proposition has it… then one would have gained the right to determine all efficient force univocally as–will to power. The world viewed from inside… it would be “will to power” and nothing else.
F.W. Nietzsche -from Beyond Good and Evil, s.36, Walter Kaufmann transl.