In many ways, physics stems from ancient Greek philosophy. From Thales ' first attempt to characterize matter, to Democritus ' deduction that matter ought to reduce to an invariant state to the Ptolemaic astronomy of a crystalline firmament, and Aristotle's book Physics (an early book on physics, which attempted to analyze and define motion from a philosophical point of view), various Greek philosophers advanced their own theories of nature. Physics was known as natural philosophy until the late 18th century.