Sorry for the dearth of postings, it's been incredibly hectic lately. These are very much thoughts in progress. The main references are to Janaway's Beyond Selflessness: Reading Nietzsche's Genealogy (OUP, 2007) and to my Nietzsche on Morality (Routledge, 2002).
Christopher Janaway claims that most Nietzsche scholars now accept that Nietzsche is a naturalist in what Janaway calls the “broad sense”:
He opposes transcendental metaphysics, whether that of Plato or Christianity or Schopenhauer. He rejects notions of the immaterial soul, the absolutely free controlling will, or the self-transparent pure intellect, instead emphasizing the body, talking of the animal nature of human beings, and attempting to explain numerous phenomena by invoking drives, instincts, and affects which he locates in our physical, bodily existence. Human beings are to be “translated back into nature,” since otherwise we falsify their history, their psychology, and the nature of their values—concerning all of which we must know truths, as a means to the all-important revaluation of values. This is Nietzsche’s naturalism in the broad sense, which will not be contested here. (Janway 2007: 34)
This is less a “broad sense” of naturalism, however, than it is “Laundry List Naturalism.” Janaway seems oddly indifferent to the question why these are a set of views a philosophical naturalist ought to hold, or what it is that makes them the views of a philosophical naturalist at all.
My aim, in earlier work, was to make some philosophical sense of why Janaway’s Laundry List Naturalism, in fact, seems descriptively adequate to many things Nietzsche says. I suggested that underlying this Laundry List Naturalism was, in fact, a kind of familiar “Methodological Naturalism” (hereafter “M-Naturalism”), according to which “philosophical inquiry…should be continuous with empirical inquiry in the sciences” (2002: 3). Many philosophers are and have been Methodological Naturalists, but to understand Nietzsche, everything turns on the precise kind of M-Naturalism at issue. I emphasized two commitments of Nietzsche’s M-Naturalism. First, I claimed that Nietzsche is what I called a Speculative M-Naturalist, that is, a philosopher, like Hume, who wants to “construct theories that are ‘modeled’ on the sciences…in that they take over from science the idea that natural phenomena have deterministic causes” (Leiter 2002: 5). Speculative M-Naturalists do not, of course, appeal to actual causal mechanisms that have been well-confirmed by the sciences: if they did, they would not need to speculate! Rather, the idea is that their speculative theories of human nature are informed by the sciences and a scientific picture of how things work. Here, for example, is Stroud’s influential formulation of Hume’s Speculative M-Naturalism:
[Hume] wants to do for the human realm what he thinks natural philosophy, especially in the person of
Newtonian theory provided a completely general explanation of why things in the world happen as they do. It explains various and complicated physical happenings in terms of relatively few extremely general, perhaps universal, principles. Similarly, Hume wants a completely general theory of human nature to explain why human beings act, think, perceive and feel in all the ways they do….
[T]he key to understanding Hume’s philosophy is to see him as putting forward a general theory of human nature in just the way that, say, Freud or Marx did. They all seek a general kind of explanation of the various ways in which men think, act, feel and live….The aim of all three is completely general—they try to provide a basis for explaining everything in human affairs. And the theories they advance are all, roughly, deterministic. (Stroud 1977: 3, 4)
So Hume models his theory of human nature on Newtonian science by aiming to identify a few basic, general principles that will provide a broadly deterministic explanation of human phenomena, much as Newtonian mechanics did for physical phenomena. Yet the Humean theory if still speculative, because its claims about human nature are not confirmed in anything resembling a scientific manner, nor do they even win support from any contemporaneous science of Hume’s day.
Nietzsche’s Speculative M-Naturalism obviously differs from Hume’s in some respects: Nietzsche, for example, appears to be a skeptic about determinism based on his professed (if not entirely cogent) skepticism about laws of nature. Yet Nietzsche, like Hume, has a sustained interest in explaining why “human beings act, think, perceive and feel” as they do, especially in the broadly ethical domain. Like Hume, Nietzsche proffers a speculative psychology, though as I have argued elsewhere (Leiter 2007; Knobe & Leiter 2007), Nietzschean speculations seem to fare rather well in light of subsequent research in scientific psychology. And this speculative psychology (as well as the occasional physiological explanations he offers in passing) appear to give us causal explanations for various human phenomena, which, even if not law-governed, seem to have a deterministic character (cf. Leiter 2002: 5).
But I also emphasized a second aspect of Nietzsche’s M-Naturalism. As I noted, some M-Naturalists demand a kind of “results continuity” with existing science: “philosophical theories,” should they believe, “be supported or justified by the results of the sciences” (Leiter 2002: 4). I argued, however, there is only one kind of “results continuity” at work in Nietzsche, namely, the result that the German Materialists of his day thought followed from advances in physiology, namely, “that man is not of a ‘higher…[or] different origin’ than the rest of nature” (Leiter 2002: 7). Arguably, Nietzsche’s one bit of Substantive (in contrast to Methodological) Naturalism--meaning “the (ontological) view that the only things that exist are natural” [Leiter 2002: 5]--is a consequence of this “results continuity. Here, of course, Nietzsche had in mind the developments in 19th-century physiology which appeared to support the view that all kinds of conscious experiences and attitudes had physiological explanations. (I discuss this at greater length in my book.)
By introducing Nietzsche’s naturalism within a broader typology of kinds of naturalism, I appear to have sowed confusion among some scholars. Janaway’s recent critique of my naturalist reading is illustrative. He complains, for example, that:
[N]o scientific support or justification is given—or readily imaginable—for the central explanatory hypotheses that Nietzsche gives for the origins of our moral beliefs and attitudes. For a prominent test case, take Nietzsche’s hypothesis in the Genealogy’s First Treatise that the labeling of non-egoistic inaction, humility, and compassion as “good” began because there were socially inferior classes of individuals in whom feelings of ressentiment against their masters motivated the creation of new value distinctions. This hypothesis explains moral phenomena in terms of their causes, but it is not clear how it is justified or supported by any kind of science, nor indeed what such a justification or support might be. (2007: 37)
Janaway, it bears noting, in fact endorses a weaker version of my reading of Nietzsche as an M-Naturalist, though the weakening seems to derive from his misunderstanding of the role of “results continuity” in my interpretation of Nietzsche’s M-Naturalism. He writes that “Nietzsche is a naturalist to the extent that he is committed to a species of theorizing that explains X by locating Y and Z as its causes, where Y and Z’s being causes of X is not falsified by our best science” (2007: 38). Janaway prefers this account, because of his doubts about whether there are actual scientific results supporting Nietzsche’s actual causal explanations. Since my reading of Nietzsche’s naturalism emphasized its speculative character, Janaway’s formulation serves as a useful way of stating a pertinent constraint on speculative explanations: namely, that they not invoke entities or mechanisms that science has ruled out of bounds. But even so, it may seem an unnecessarily weak a criterion: why not expect, instead, that a good speculative naturalist will rely on explanatory mechanisms that enjoy some evidential support, or that enjoy a wide explanatory scope, of the kind we expect genuine explanations in the sciences to exemplify? I do not think there is text in Nietzsche that settles this matter, and so this is more a matter of giving the most philosophically appealing reconstruction of his actual argumentative and explanatory practice.
 Janaway (2007: 37) says: “the status of this as a ‘result’ is perhaps debatable: it is hard to say whether the exclusively empirical nature of humanity was a conclusion or an assumption of scientific investigation in the nineteenth century or at any time.”. This I find extraordinary. If one discovers that conscious experiences have a neurophysiological explanation, or an explanation in terms of the biochemistry of the brain, hasn’t one adduced some evidence that bears on whether man is of a “higher or different origin” than the rest of nature? Our consciousness and our capacity for self-reflection, for spirituality, for “inwardness” are all among the typical phenomena appealed to as evidence of our “higher” or “different” nature, perhaps as glimpses of our immaterial “soul” even. If, in fact, they are explicable through processes and mechanisms that are operative in other parts of the natural world, is that not evidence that we are not of “a higher or different origin” than other natural things?