Man is unique in that he does not have to live only in the present. His world is extended by symbolic thought to include past history and future time. He can read about the extinct civilization of the Inca, and using scientific knowledge, speculate as to the nature of the universe. Of course, no modern man has witnessed the Inca civilization or touched a star, yet he can talk about them knowingly because of symbols. The planet Pluto, in fact was first discovered mathematically by scientists long before its existence was actually verified visually by means of high powered telescopes.