The term ‘milieu’ traverses the depth-psychology corpus along two distinct but interrelated axes. The first is biological and phenomenological: Merleau-Ponty, as reconstructed by Evan Thompson, deploys milieu as the relational field co-constituted by organism and environment through dialectical, bi-directional exchange — not a backdrop against which life unfolds, but the very product of living activity. Gilbert Simondon radicalizes this further, treating milieu as the ontological medium through which individuation proceeds: neither a container nor a fixed surround, but ‘the very activity of relation,’ a pre-individual charge from which individual and milieu simultaneously crystallize. The second axis is literary and social: Balzac, as analyzed by Auerbach, wields milieu as a narrative and theoretical hinge — the interpenetrating determination between person and environment that lends realistic fiction its sociological depth (‘sa personne explique la pension, comme la pension implique sa personne’). Jung and Stein invoke milieu in a more psychological register, noting how alterations of milieu produce striking alterations of personality, gesturing toward the situational constitution of character. Across all these deployments, milieu is never static: it names a dynamic, reciprocally determining relation between subject and surround — whether biological, psychical, social, or literary — and thus stands as one of depth psychology’s master-concepts for theorizing the embeddedness of the self.