All phenomena exist in one of three possible forms of definedness, which are: 1) divergence, 2) convergence, and 3) suspension. While environments and inanimate objects like planet, stars, rocks, weather, and grapes have no control over their definedness, phenomena with agency, such as me and you, do. Though it varies agent-to-agent, humans evolved an intricate neurobiological compass designed and optimized to navigate definedness into these three forms. It allows us to automatically move through existence with ease. It is called The Definedness Detector, and it resides deep in the human brainstem: locus coeruleus (LC). In this paper, I'll unveil how this The Definedness Detector works, explore properties, and demonstrate how understanding it transforms our unpredictable, but endlessly mystifying ride through existence from unknowable to unforgettable.