The Greeks invented philosophy. The word itself comes from the Greek, meaning "love of wisdom." From the sixth century B.C.E. until the fourth century B.C.E., Greek thinkers throughout the Hellenic world started addressing a wide range of questions about the nature of the universe and the meaning of life. It is a mystery why the Greeks, beginning around the sixth century B.C.E., were the first to pursue sustained philosophical inquiry. But since they lacked palace bureaucracies or professional priests, no vested interests stood opposed to free speculation and observation. In addition, because literacy was widespread—owing to the simplified Greek alphabet—and because new military tactics and governmental institutions gradually strengthened their ideals of equality, the Greeks came to believe that they were different from others, especially their Persian neighbors, and began ever more to speculate on the reasons for the differences. Finally, their relative indifference to material needs and the availability of leisure time offered the Greeks unprecedented opportunities for reflection.