Psychological phenomena take place at the border between person and environment. Indeed, psychology as a whole may be seen as a science of human liminal constructions, a science concerned with the dynamic relationships that exist between people and what surrounds them and with the constant border crossing that defines the arena within which all human development takes place. From this perspective, the central question becomes: how do humans deal with such transitions throughout the course of their lives? Cultural psychology offers a way of addressing, theoretically and empirically, the epistemological and social dimensions of this question. In addition, we argue that mereotopology—the theory of the relations of part to whole and of part to part within a whole—provides a conceptual framework of enormous potential for appreciating its unexplored ontological underpinnings, thus contributing to the foundations of psychology as a developmental science of the inherent uncertainty that accompanies all individual and social becoming.