Chapter one of STP (delivered 11 Jan. 1978) discusses the milieu in the context of the town. How do you plan a town, and how do you lay it out spatially?
Under sovereignty and under discipline these questions had different answers. They did not go away but were joined by a different emphasis, that of security.
First, there is no longer any question of construction within an empty or emptied space, as in the case of those, let’s say, disciplinary towns such as Richelieu, Kristiania, and suchlike. Discipline works in an empty, artificial space that is to be completely constructed. (STP: 19)
Security will rely on a number of material givens. It will, of course, work on site with the flows of water, islands, air, and so forth. Thus it works on a given. (STP: 19)
Continuing from this, security deals with the population and circulation.They are not known precisely, they are changing and dynamic, so this brings in the question of probabilities. This much has been described before.
But what is interesting here is that this is conceived by Foucault as a milieu, in terms that go back to one of his first works, History of Madness. In STP this is drawn from Lamarck, and in HoM it is drawn from Buffon.
The space in which a series of uncertain elements unfold is, I think, roughly what one can call the milieu. As you well know, the milieu is a notion that only appears in biology with Lamarck… What is the milieu? It is what is needed to account for action at a distance of one body on another. It is therefore the medium of an action and the element in which it circulates. It is therefore the problem of circulation and causality that is at stake in this notion of milieu. (STP: 20-1)
The milieu is a causal factor then for influencing bodies. Foucault’s notion here was derived from Canguilhem according to a footnote (fn. 37). (Canguilhem drew it via Buffon from Lamarck.)
By contrast, in HoM, MF speaks of “penetrative forces” (p. 365):
To the classical mind, madness could easily be the effect of an external “milieu,“…
But a new notion was to emerge…it is as though what emerged from the cosmic whole and its seasonal stability was an independent, relative and mobile element, subject to constant progression and continuous acceleration, whose task it was to explain this incessant multiplication, the grand contagion of madness…
Perhaps we should allow this notion [of milieu] which had not yet found its equilibrium or its final denomination, to retain its unfinished nature, and speak instead, like Buffon of “penetrating forces,” which allowed not only for the formation of an individual, but also the appearance of the different varieties of human species: the influences of climate, of nutrition and of the way of life. This negative “differential” notion first appeared in the eighteenth century, to explain variations and diseases rather than adaptations and convergences. It was as if the “penetrating forces” formed the converse, or the negative, of what was later to become the positive notion of the milieu (HoM: 364-5)
This is close to environmental determinism as a causal factor in shaping “varieties” of human beings. In this case Foucault is talking about 18th C. conceptions of subject formation in regards to madness. But for Buffon this also applied to the formation not of different races or species, but of different populations.