In this study I have outlined the grounds of a new theory, to explain the mind-brain-body triad in health and disease. This is the psychomotor theory. First, I have briefly analyzed the historical development of the relationship between mind and brain, which is being discussed since more than a few thousand years. The tight junctions between psychological and motor systems were subjected to a detailed analysis using examples in health and disease. The feedback circuits between mind, brain, and body were shown to occur within the mind-brain-body triad, in normal states, and psycho-neural diseases. It was stated that psychiatric signs and symptoms are coupled with motor disturbances; neurological diseases are coupled with psychological disturbances; changes in cortical and spinal motor-system activity may influence mind-brain-body triad, and vice versa. Accordingly, a psychomotor theory was created to explain the psychomotor coupling in health and disease, stating that, not the mind-brain duality or unity, but the mind-brain-body triad as a functional unit is essential in health and disease, since mind does not end in the brain, but further controls movements, in a reciprocal manner; mental and motor events share the same neural substrate, cortical and spinal motor neurons; mental events emerging from the motoneuronal system is strongly coupled with the unity of the mind-brain-body triad. So, the psychomotor theory rejects the mind-brain duality, instead, advances the unity of the psychomotor system, which will have important consequences in understanding and improving the human mind, brain, and body in health and disease.