While Hazlitt's Lesson is not exactly a separate economic principle, it is a form of mental discipline that must be exercised when constructing or analyzing policy. The Gwartney and Stroup book provides excellent examples of how rent control laws destroy urban housing, protective tariffs and quotas harm American consumers and workers, and jobs programs fail to create a single job overall. Hazlitt's book, Economics in One Lesson, is still one of the best examples of the economic way of thinking put into practice. More often than not, the harm of these policies falls not only on people who were never considered at the time, but on the supposed beneficiaries of the programs. Debaters and extempers are advised to spend time with each of these books to get a feel for the way Hazlitt's Lesson is put into practice.
Examples are the best teacher, here.
Opportunity cost reminds you that when government creates a beneficiary somewhere there is a cost being paid. The idea of substitutes reminds you that people are born circumventers -close off one avenue or make it more costly, and they will leave that activity and switch to another. Competition reminds you that helping one group harms others that compete with that group. Today there is great concern about the ease with which medical records can be accessed through computerized networks by people who have no business knowing this sensitive information. Confidential medical information is gathered and consolidated into vulnerable centralized databases partly as an unintended consequence of government policy concerning health insurance.