… is from page 117 of the 1992 collection of some of William Graham Sumner’s best essays, On Liberty, Society, and Politics  (Roger C. Bannister, ed.); specifically, this quotation is from Sumner’s January 1881 Princeton Review essay, “The Argument against Protective Taxes”:
The notion that the legislature has a wisdom greater than that of the people, and can point out the industries they ought to pursue, has often been refuted; but the protective theory really assumes more than that. It assumes that the law can enlighten the desire for profit, and make it a more trustworthy guide than it would be under freedom. In truth there is nothing at all wanted in the cases to which protection is applied but capital, which the law can never produce. The efficiency of the tariff is that it does get this capital – from other people.