 Schopenhauer further comprehends these three (and for him, interdependent) principles as expressions of a single principle, namely, the principle of sufficient reason, whose fourfold root he had examined in his doctoral dissertation. In The World as Will and Representation, Schopenhauer often refers to an aspect of the principle of sufficient reason as the “principle of individuation” (principium individuationis), linking the idea of individuation explicitly with space and time, but also implicitly with rationality, necessity, systematicity and determinism. He uses the principle of sufficient reason and the principle of individuation as shorthand expressions for what Kant had more complexly referred to as space, time and the twelve categories of the understanding (viz., unity, plurality, totality, reality, negation, limitation, substance, causality, reciprocity, possibility, actuality [Dasein], and necessity).