Mental Gravity is a novel theory postulating that the mind actively simulates properties of physical gravity under different conditions, including when depressed. It describes how aspects of the brain, body, and self display characteristics analogous to the experience of gravitational force and underlying spacetime curvature as conceived in the general theory of relativity. It is systematically related to the Bayesian active inference account of brain behaviour, specifically neuronal gauge theories. Mental Gravity explains how “gravity priors” (i.e., the intuitive physics of heavy and falling objects) is mobilised for social/emotional communication, and also how it relates to autobiographical memory and the self. The theory models the interaction between present (experiential) and past (autobiographical) poles of the so-called “self-axis” as analogous to the interaction between two masses in a gravitational field. Depression represents a particularly salient form of Mental Gravity where the past self dominates present experience, leading to quasi-delusion beliefs or quasi-hallucinatory perceptions of increased gravity leading to gravity-like experiences of feeling down, low, heavy, and slow. A plausible neurobiological substrate for Mental Gravity is also proposed (relativistic pseudo-diffusion) to connect to the experience of subjective time dilation (slowing) in depression to the slowing/curavture of neural spacetime dynamics. Using a “common currency” approach, I therefore propose that Mental Gravity is fundamental to: 1) phenomenological experience of the self in space and time; 2) spatial and temporal perception and cognition; and 3) neurobiological spatiotemporal dynamics.