The theory of Mental Gravity explains what it means to be psychologically “down” and how mental downness manifests in various aspects of cognition and behaviour, emotion and personality, the brain, phenomenology, and psychopathology. The theory contends that Mental Gravity is experienced as a simulation of (increased) physical gravity that communicates different mental states associated with being down, low, heavy, and slow, including when depressed. The simulation relates to core aspects of autobiographical memory, selfhood, and subjective space and time. The theory proposes that the intuitive physics of physical gravity (i.e., the “internal gravity model”) is used as a mental “template” for behaviours that communicate social or emotional gravity in particularly solemn, serious, or grave situations. Mental Gravity thus draws a behavioural analogy between the physical and social-emotional environments. At a deeper level, the theory also describes the underlying neural, cognitive, and phenomenological processes driving the mental simulation of physical gravity in terms of fundamental physics. Neural correlates are proposed in the vestibular, salience, default mode, and central executive networks. On the basis of default mode activity, the self is viewed as the brain’s cognitive “centre of mental gravity” constituting an accumulated “mass” of autobiographical memory. Phenomenologically, the theory uses gravity to model the interaction between the internal self and the external world. Depression represents severe Mental Gravity where persistently strong feelings of increased physical gravity are underscored by more drastic changes in the sense of self, seclusion from the world, and distorted subjective experience of space and time.