In "A Conflict of Visions", Thomas Sowell proposed that the fundamental difference between the policies of the left and the right derive from their respective views of human nature.
The left sees man in general as perfectly malleable. It sees every individual's problems as being caused by society as a whole. Criminal behavior under this theory is merely a response to injustice; poverty is a condition brought on by greed; depression, drunkenness and illness are all seen as a fault of the medical system or our general "awareness". Since individual problems are the fault of the whole of society, the solution must be to fix society by massive government intervention.
People on the right take an inverse view of the situation. Conservatives believe in individual responsibility. This means, if someone commits murder, he is bad. If someone is poor he has declined to take advantage of opportunities manifest within a free market system. If someone is uneducated, he has not worked hard enough to secure education for himself. This attitude among conservatives means that the perceived solution is not to change society in a general way but to get government out of the business of regulating the people in mass and making them take responsibility for their actions in particular. Social man then is not malleable, but the individual can be guided by market forces.
Conservatives believe that the history of the last 40 years is stark evidence that the leftist view is complete bunk. The failure of the welfare system put in place during the Johnson administration, the price controls of the Nixon administration and the general malaise caused by the various programs implemented by the Carter administration provide more than sufficient evidence. Couple this with the success of welfare reform in the 1990s and it is hard to deny that Thomas Sowell was correct in his appraisal of the diametrically opposed views of the world espoused by the right and left.