This paper analyses chapter 58 of Theophrastus’ De sensibus , where Democritus’ account of phronein is famously presented. Democritus traces phronein to symmetria of the soul, that is conceived, in turn, as a state of thermic equilibrium, depending on his consideration of psyche as an aggregate of spherical and thin atoms flowing throughout the body and giving it life, movement, and perception. As a consequence, according to him, psychic states go hand in hand with changes in the body. In the following section of this paper, I offer additional evidence in favor of the controversial manuscript reading μετὰ τὴν κίνησιν. As a result, a consistent theory by Democritus can be reconstructed, according to which the balanced state of mobility and temperature of the psychic atoms concentrated in the head that generates a correct thinking necessarily follows the movement of sensible eidola throughout the body. In fact, a number of passages from Hippocratic writings attest that Democritus shared the key features of this explanation with the physiological and medical approach of the time.