The essence of human nature is its perpetually evolving desire for pleasure. To realize this desire, we feel compelled to discover, invent, and improve our reality. The gradual intensification of the desire for pleasure has been the force behind human evolution throughout our history.
The desire for pleasure evolves through several stages. In the first stage, it manifests in the need for sustenance, such as food, sex, and family. In the second stage, the desire for wealth arises, and in the third, there is a craving for honor, power, and fame. Development of these three stages had lead to major changes in human society – it became a diversified, multi-class society.
Courtesy of http://www.youtube.com/watch?v=uRxmCRFmkTU