In high school I read this book called "A Study of History" by Arnold Toynbee, and in the book he said that civilizations define themselves, grow and change via a series of challenges that are presented to them. For instance, in ancient Greece, Sparta and Athens both faced the challenge of a rising population. Sparta got the food for their people by conquering their neighbors, and Athens sent the 'surplus' people abroad to form colonies and relied on (more or less) a free market to run things...