Sowell argues that people tend to have one of two visions of life. Holders of the constrained vision--roughly, modern conservatives--believe that perfection in social and economic affairs is impossible. Why? For one thing, the knowledge needed for such perfection can't be obtained by one person or even by a small group of people. So constrained vision holders concentrate on the margin: they look for policies to make things a little better rather than a little worse.
Holders of the unconstrained vision--roughly, modern Liberals--believe that it is not only possible to achieve perfection in social and economic affairs but that people of intelligence and good will can absolutely devise the policies to get us there. So Liberals find people who quarrel with or oppose their policies utterly baffling. How could anyone reject perfection? They can only believe such people are immoral and evil.
Many of the big political debates in my lifetime are well described by Sowell's brilliantly simple scheme.