As human society has developed from its earlier, more primitive state to the advanced civilizations of today, men have learnt to analyse their own processes of body and mind and their natural environment in ever greater detail and to control and use natural forces for their own ends. Philosophers, prophets, and artists have given men deeper insight into religion and morals, human emotions, and the life of society. Men have organized themselves in groups such as families, clans, and nations and have developed elaborate systems of law and political and social institutions. Trade and industry have played an ever increasing part in their lives. This long process has been haphazard and its various aspects have taken place at vastly different speeds. There is no direct and steady progression from falsehood to truth, from the dug-out canoe to the space capsule, from the cave-man family to the modern industrial state. What is clear, however, is that men's ideas in the course of the long centuries have grown more and more complex and varied; and that is what chiefly concerns us here.