This book offers a scholarly audience a new key to Gilbert Simondon’s encyclopaedic philosophy. It traces a psychological theory in Simondon’s work that guides his entire oeuvre and that holds together his technological and philosophical interests. Against the dominant view that psychology occupies a marginal place in Simondon’s thought, the volume demonstrates that genetic psychology constitutes its methodological and theoretical core. Far from being a secondary discipline, psychology functions as a transversal matrix that articulates ontology, technology, life, and collective existence within a unified theory of individuation. By reconstructing the genesis of this project — from early cybernetic writings to the two doctoral theses — the book shows that their apparent bifurcation conceals a deeper unity grounded in a general allagmatics of being. Drawing on lesser-known texts and lectures, it redefines imagination, affectivity and creativity as ontogenetic vectors and repositions Simondonian psychology as an open paradigm for rethinking subjectivity, technology, and collective life in the digital age.