American traditions and the American ethic require us to be truthful, but the most important reason is that truth is the best propaganda and lies are the worst. To be persuasive we must be believable; to be believable we must be credible; to be credible we must be truthful. It is as simple as that.
Bio: Alfred Marshall was one of the most influential economists of his time. His book, Principles of Economics, was the dominant economic textbook in England for many years.