This chapter is devoted to On Regimen, the longest and by far most complex work by a cosmological doctor to have survived from the Classical period. Topics discussed include the “self-sufficiency” (autarkeia) of fire and water, the pendular movement of various cycles, the two-way “resemblance” (apomimesis) between the body and the cosmos, and the use of both cosmological principles and “prodiagnosis” as a solution to the problem of individualization. This chapter also considers On Regimen’s theories about the soul, which is concentrated in the sun and separates off into other, smaller souls to give movement, life, intelligence, and divinity to everything it inhabits.