and attracts new people to that line of production. An excess supply of a product (more of the product than people are willing to buy) drives prices down, and producers refocus energy and money to other areas where there is a need. Of the Sources of the General or Public Revenue of the Society: In his discussion of taxes in Book Five, Smith wrote: He also introduced the distinction between a direct tax, and by implication an indirect tax (although he did not use the word "indirect"): And further: This term was later used in United States, Article I, Section 2, Clause 3 of the U.S. Constitution, and James Madison, who wrote much of the Constitution, is known to have read Smith's book. Of War and Public Debts: Smith then goes on to say that even if money was set aside from future revenues to pay for the debts of war, it seldom actually gets used to pay down the debt. Politicians are inclined to spend the money on some other scheme that will win the favour of their constituents. Hence, interest payments rise and war debts continue to grow larger, well beyond the end of the war. Summing up, if governments can borrow without check, then they are more likely to wage war without check, and the costs of the war spending will burden future generations, since war debts are almost never repaid by the generations that incurred them. Reception and impact Greater Britain Intellectuals, critics, and reviewers The first edition of the book sold out in six months. The printer William Strahan wrote on 12 April 1776 that David Hume said The Wealth of Nations required too much thought to be as popular as Edward Gibbon's The History of the Decline and Fall of the Roman Empire. Strahan also wrote: "What you say of Mr. Gibbon's and Dr. Smith's book is exactly just. The former is the most popular work; but the sale of the latter, though not near so rapid, has been more than I could have expected from a work that requires much thought and reflection (qualities that do not abound among modern readers) to peruse to any purpose." Gibbon wrote to Adam Ferguson on 1 April: "What an excellent work is that with which our common friend Mr. Adam Smith has enriched the public! An extensive science in a single book, and the most profound