"Robert Grossetest... described a compelling, unified version of the universe in which light was the fundamental force. Differing from Aristotle and others who had argued that heavenly bodies consisted of some pure, quintessential substance unlike any found on Earth, Grosseteste maintained that the stars themselves are composed of the four earthly elements. Like the attraction of a lump of iron to a magnet, light from the stars attracts comets, which themselves are a form of purified fire. His model also explained the moon's influence on the tides in terms of the way its light causes water to swell and move upward."