If all this is true, it would be as silly and immoral to "let nature take her course" as for a physician to neglect a case of pneumonia. It would also be a libel on economic science, which has its therapeutics as truly as medical science.--Irving Fisher (1933) "Debt-Deflation Theory of Great Depressions," Econometrica (347).
Fisher wrote these lines at the height of the great depression just as the age of antibiotics was commencing and so, thereby, the possibility of reliable curative treatment of pneumonia. For, Fisher the analogy between physician and economist does not reside in a do-no-harm-principle, but rather in the moral command to "gladly share such knowledge" as will aid the patient, that is society, when one has the means to do so. That is to say, in Fisher's hands the economist-as-physician metaphor is a moral call to political action (in fact to reflate the dollar in order to stop the vicious spiral of debt-repayment and deflation/high unemployment/debt-trap, familiar to modern reader from the Greek situation).
Fisher is not the first to use the economist-as-physician metaphor. As Jason Stanley has noted in a different context, the trope goes back at least to Plato (recall). It was undoubtedly revived in the hands of the eighteenth-century physiocrats who often literally combined medical practice and being an economists (cf. Quesnay). Even so, Fisher's piece is the first modern use of the metaphor within economics I am familiar with. (To the best of my knowledge, utilitarians did not conceive themselves as physicians of society.) Obviously, I welcome correction by my readers.*
One admirer of Fisher, qua mathematical economist (bot not qua eugenicist) was Milton Friedman, who called Fisher "the greatest economist that the United States has ever produced." Friedman certainly knew the Debt-Deflation piece well (see here), and was, thus, familiar with the passage quoted above. Perhaps it inspired him to embrace the economist as physician metaphor in his writing, including in a piece, "the cause and cure of inflation," in which he sustains the metaphor throughout. The most famous use by Friedman is offered in the context of defending his (quite modest, actually, but politically non-trivial) involvement in offering economic advice to Pinochet. (I have discussed the episode here.)
In spite of my profound disagreement with the authoritarian political system of Chile, I do not consider it as evil for an economist to render technical economic advice to the Chilean Government, any more than I would regard it as evil for a physician to give technical medical advice to the Chilean Government to help end a medical plague. (Newsweek, June 14, 1976)
Friedman implies that sharing technical, economic knowledge is always moral. Perhaps, Friedman thinks it is moral to to dispense treatment that only benefits the members of the Chilean Government (which, let's stipulate, were seriously immoral); the morality of such treatment has been explored in Night Train to Lisbon by Pascal Mercier. Friedman ignores here how the experts' incentives may corrupt his judgment. Unlike physicians, economists still do not have a code of ethics, although George DeMartino is developing the intellectual framework for it (recall). (We philosophers are no better.) But it is more likely that the main justification of Friedman's position is meant to be the more utilitarian thought that the cure has good consequences not so much for the Chilean Government, but for the majority of Chileans citizens who suffer from bad economic policies (analogous to ending "the medical plague"). It does not follow, however, that there is always a duty to dispense treatment; Friedman does not imply, for example, that it would be wrong to neglect societies that can benefit from his cures. His use of the economist as physician metaphor is not a call to action.