The principium individuation is is a notion from the philosophy of Arthur Schopenhauer.
He defined it as space and time, noting that he had borrowed the expression from scholasticism.
The principium individuationis was the possibility of multiplicity (The World as Will and Representation [1819 ], trans. E. J. Payne, 2 vols. [New York: Dove1J, pp. 145-46).
The term was used by Eduard von Hartmann, who says its origin in the unconscious.
It designated the “uniqueness” of each individual set against the “all-one unconscious” (Philosophie des Ubewussten: Versuch ein Weltanshanuug [Berlin: C. Dunker, 1869], 519 ).
In 1912, Jung wrote,
“Diversity arises from individuation. This fact validates an essential part of Schopenhauer’s and Hartmann’s philosophy in profound psychological terms” (Transformations and Symbols of the Libido, CW B, § 289).
In a series of papers and presentations later in 1916, he developed his concept of individuation (“The Structure of the Unconscious,” CW 7, and “Individuation and Collectivity,” CW 18) (see introduction).
In 1921, he defined it as follows:
“The concept of individuation plays no minor role in our psychology. Individuation is in general the process of the formation and particularization of individual beings; especially the development of the psychological individual, as a being distinct from generality, from collective psychology. Individuation, therefore, is a process of differentiation, having for its goal the development of the individual personality” (Psychological Types, CV! 7, § 758). ~The Black Books, Vol. V, Page 271, fn 389