Within the depth-psychology corpus, ‘baby’ functions not merely as a developmental subject but as the primary site at which foundational psychic structures — trust, separation, object relations, self-formation — are first constituted or foreclosed. Winnicott’s contributions are axial: the baby is the locus of potential space, transitional phenomena, and the paradox of separation-yet-merger with the mother, whose reliable presence or absence determines whether a capacity for creative living can be established at all. Klein reads the baby as already engaged in object relations from birth, subject to persecutory anxiety, splitting, and the earliest formations of the good-and-bad breast that will underwrite later character. Bowlby and attachment theorists approach the baby as a regulatory system whose protest at separation, stranger anxiety, and set-goal seeking are biologically grounded indices of bond quality. Post-Jungian writers such as Samuels, following Fordham, situate the baby as bearer of an a priori self that deintegrates to meet the environment. The somatic and developmental trauma traditions — Winhall, Levine, Dayton, Maté — use the baby to anchor arguments about early nervous-system patterning, attunement failure, and the embodied origins of later dysregulation. Across all these lineages, the baby stands as the irreducible empirical case against purely intrapsychic theory: what happens to it matters, constitutively and lastingly.