Let's start with section 9, which mocks the Stoic claim to live "according to nature," accusing the Stoics of, in effect, projecting their values onto nature:
Your pride wants to dictate and incorporate your morals and ideals into nature--yes, you want to make all existence exist in your own image alone--as a huge eternal glorification and universalization of Stoicism! For all your love of truth, you have forced yourselves so long, so persistently, and with such hypnotic ridigity to have a false, namely, Stoic, view of nature, that you can no longer see it any other way....But this is an old, eternal story: what happened back then with the Stoics, still happens today, just as soon as a philosophy begins believing in itself. It always creates the world in its own image, it cannot do otherwise; philosophy is this tyrannical drive itself, the most spiritual will to power, to the "creation of the world," to the causa prima.
Maudemarie Clark argued powerfully in her 1990 book that in reading what Nietzsche says about will to power, we must remember the charge he leveled against the Stoics, and that should make us hesitant to interpret Nietzsche as really intending the will to power as a metaphysics of nature, lest he simply be replicating, unselfconsciously, the Stoic mistake.
How then should we understand the claim in BGE 13 that, "a living thing wants to discharge its strength--life itself is will to power--self-preservation is only one of the indirect and most frequent consequences of this." Does this involve Nietzsche in the Stoic mistake? I had been inclined to think so, but was convinced otherwise by the discussion in the reading group. Notice, first, and crucially, that BGE 13 concerns Leben, while BGE 9 is quite clearly mocking the Stoic's claims about Natur. Indeed, even BGE 9 offers its own account of Leben:
[I]sn't that [Leben] wanting specifically to be something other than this nature [where nature is said to be "profligate without measure, indifferent without measure, without purpose and regard, without mercy and justice" etc.]? Isn't living assessing, preferring, being unfair, being limited, wanting to be different?
One might think, of course, that the latter are attributes consistent with life also being will to power. The crucial point, then, is that BGE 13's claim is not about nature, but about living things (esp. humans), and its epistemic status differs from that of the Stoic projection of its morality onto nature precisely in that it is proferred as part of an inference to the best explanation of observable life, in particular, namely, that it is justified by Ockham's Razor as superior to the claim that life is essentially about self-preservation.
This way of taking BGE 13, and distinguishing it from BGE 9, also fits nicely with the main point of the preceding section 12. That section attacks the "atomistic need," particularly in psychology, in which, following Christianity, one takes "the soul" to be "something indestructible, eternal, indivisible...a monad" (BGE 12). Against this, Nietzsche wants to make room "in the realm of science" for more sophisticated hypotheses, such as "the soul as subject-multiplicity" or the "soul as a society constructed out of drives and affects," the latter being a recognizable Nietzschean hypothesis and not only in BGE.
If Christian simplicity about the soul was a mistake, so too is its opposite: "there is absolutely no need to give up 'the soul' itself, and relinquish one of the oldest and most venerable hypotheses--as often happens with naturalists: given their clumsiness, they barely need to touch 'the soul' to lose it." Clumsy readers of the naturalistic reading of Nietzsche sometimes quote this out of context thinking it an objection to my view, but not realizing what it means: for it is quite clearly poking fun at one very particular naturalist, Ludwig Buchner, whose Kraft und Stoff advances a kind of eliminative materialism about the "soul," equating it with the neurophysiology of the brain. (This isn't the only place in the book he chastises Buchner--in sec. 204, he dismisses the "old doctors" who think physiology can replace philosophy for failing to recognize the crucial role of legislating values that philosophers perform.) Section 12, in short, is a defense of the autonomy of psychological explanation, against religious simplifications and eliminative materialism--hardly surprising, of course, in a Chapter that concludes by affirming that "psychology is again the path to the fundamental problems (BGE 23). And, of course, on my naturalistic reading, the autonomy of psychological explanation is crucial.
Section 13, then, follows this defense of the autonomy of explanation in a psychological idiom by offering a very general hypothesis: contrary to those who think the primary psychological motive is self-preservation, it is really will to power. This is not moral projection masquaering as metaphysics of nature; this is psychology, freed of Christian and eliminative materialist prejudices, asserting itself.
Notice that this way of reading things allows us to save Nietzsche from what I have called "the crackpot metaphysics" of the will to power in the Nachlass and still acknowledge the correctness of Clark's original hypothesis that a metaphysical reading of the will to power would be in tension with Nietzsche's criticism of the Stoics.
(I'll have more to say about Nietzsche and Buchner, especially with respect to a surprising debt to Buchner uncovered by Galen Strawson. But as readers of Nietzsche know, his vitriol is often a case study in the narcissism of small differences--think of his at times schizophrenic attitude towards Socrates, towards Spinoza, and towards Schopenhauer.)
(Note to readers: The "u" in Buchner should have an umlaut, but I can't insert it in this software.)