This is a brilliant book with an original theory, well explained with many good examples from the works of Adam Smith, Edmund Burke, Condorcet, William Godwin, Thomas Malthus and Fritz Hayek, plus others, whose political views may be understood to flow from conflicting visions of man and society.
The conflicting visions of man and society are the constrained and the unconstrained visions of human nature. The constrained vision of human nature says that man's nature limits what can be done to change him or his society. The unconstrained vision of human nature says that man can be comprehensively improved by social action and moral education: improvement is limited only by effort, not by innate human qualities or by social dynamics. In the constrained vision, the proper method to improve man is to use economic incentives and strict and consistent laws, which limit the harm men can do to each other. In the unconstrained vision, one can legislate for a better society or improve men simply by changing their environment sufficiently.
These basic visions inform consistently-opposed political theories in regard to justice, power, law, the economy, rights, warfare, punishment and rationality, etc, though few people express a pure constrained or unconstrained vision.
A significant asymmetry of moral judgments between the two visions is that those with the constrained vision (conservatives, for example) generally think their opponents are clever and sincere but misguided while those with the unconstrained vision (progressives, for example) generally condemn their political enemies as morally repugnant.
This consequence of the theory perfectly fits my experience, so although Thomas Sowell is scrupulously fair to both visions, to my mind he cannot help formulating good arguments for the rationality and truth of the constrained vision.