In Thomas Sowell's "A Conflict of Visions" he makes it clear from 2 centuries of philosophical writings that two opposing views of human nature underlie the philosophical differences between Conservatives and Progressives. Conservatives see the forces of human nature as unchanging over the history of man. Progressives believe that human nature naturally develops into a more civic minded responsible form if the person is free from the want of basic necessities.
Conservative policies are designed to work with human nature, whereas Progressive policies are intended to nurture improved human nature.
Read Sowell's book for more. Also see A Brief History of Political Evolution in America at http://cedarstrip.wordpress.com/