The article discusses the arguments with which Kepler demonstrates what we will call the fundamental theorem of optics. According to this theorem, a homocentric beam of light that passes through a transparent sphere, provided that the amplitude of the beam is small, is concentrated again at one point on the other side of the sphere. We show how the philosopher, using geometric analogies as paper tools and despite relying on both an imprecise law for refraction and certain approaches, came to an outcome that tradition incorporated as promising. Indeed, modern tradition incorporated the theorem by correcting what, in his view, were methodological errors.
