Students of social science, must fear popular approval: Evil is with them when all men speak well of them. If there is any set of opinions by the advocacy of which a newspaper can increase its sales, then the student who wishes to leave the world in general and his country in particular better than it would have been if he had not been born, is bound to dwell on the limitations and defects and errors, if any, in that set of opinions: and never to advocate them unconditionally even in ad hoc discussion. It is almost impossible for a student to be a true patriot and to have the reputation of being one in his own time.The source of the quotation is "In Memoriam: Alfred Marshall," a speech given by A.C. Pigou in 1924 and published as part of a Memorials of Alfred Marshall volume in 1925 (pp. 81-90). The quotation attributed to Marshall appears on p. 89.
A number of interesting questions lurk here. Does an economist have a duty, to self and to society, to play the role that Marshall describes in public discourse? What about in ad hoc discussion? Is that duty appropriately called "patriotism"? It does seem to me that if you find yourself in a specific setting where you know more, there is some ethical or moral duty not to use your knowledge to take undue advantage of others. This applies to all professions--car mechanics and sommeliers, as well as financial advisers and economists. But it also seems to me that people wear different hats at different times, and a commitment to live all aspects of one's life enunciating "the limitations and defects and errors" of popular opinions seems to exalt an attitude of grumpy oppositional monasticism, which may not be a utility-maximizing way for economists (or anyone else) to live.