Aristotle (384–322 BC) was the first known scientist to use various theories and observational facts to employ a consistent, structured cosmological theory of comets. He believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the zodiac and vary in brightness over the course of a few days. Aristotle's cometary theory arose from his observations and cosmological theory that everything in the cosmos is arranged in a distinct configuration. Part of this configuration was a clear separation between the celestial and terrestrial, believing comets to be strictly associated with the latter. According to Aristotle, comets must be within the sphere of the moon and clearly separated from the heavens. Also in the 4th century BC, Apollonius of Myndus supported the idea that comets moved like the planets. Aristotelian theory on comets continued to be widely accepted throughout the Middle Ages, despite several discoveries from various individuals challenging aspects of it.