“If the physician clearly perceives what is to be cured in diseases, that is to say, in every individual case of disease (knowledge of disease, indication), if he clearly perceives what is curative in medicines, that is to say, in each individual medicine (knowledge of medical powers), and if he knows how to adapt, according to clearly defined principles, what is curative in medicines to what he has discovered to be undoubtedly morbid in the patient, so that the recovery must ensue - to adapt it, as well in respect to the suitability of the medicine most appropriate according to its mode of action to the case before him (choice of the remedy, the medicine indicated), as also in respect to the exact mode of preparation and quantity of it required (proper dose), and the proper period for repeating the dose; - if, finally, he knows the obstacles to recovery in each case and is aware how to remove them, so that the restoration may be permanent, then he understands how to treat judiciously and rationally, and he is a true practitioner of the healing art.”

Schizophrenia: The Sacred Symbol of Psychiatry (1979)

Source: Invisible Monsters

The Way to Peace, Power and Long Life (1945), p. 121 (2001 edition)

" The Last Leaf http://www.classicshorts.com/stories/lastleaf.html"
The Trimmed Lamp (1907)

Ancient Medicine
Context: Certain s and physicians say that it is not possible for any one to know medicine who does not know what man is, and that who ever would cure men properly, must learn this in the first place. But this saying rather appertains to philosophy, as Empedocles and certain others have described what man in his origin is, and how he first was made and constructed. But I think whatever such has been said or written by sophist or physician concerning nature has less connexion with the art of medicine than with the art of painting. And I think that one cannot know anything certain respecting nature from any other quarter than from medicine... Wherefore it appears to me necessary to every physician to be skilled in nature, and strive to know... what man is in relation to the articles of food and drink, and to his other occupations, and what are the effects of each of them to every one.<!--pp. 174-175

Demetrius, sec. 1
Parallel Lives

Les médecins administrent des médicaments dont ils savent très peu, à des malades dont ils savent moins, pour guérir des maladies dont ils ne savent rien.
This attribution to Voltaire appears in Strauss' Familiar Medical Quotations (1968), p. 394, and in publications as early as 1956 http://books.google.pt/books?id=lCtCAQAAIAAJ&q=%22Doctors+are+men+who+prescribe+medicine+of%22&dq=%22Doctors+are+men+who+prescribe+medicine+of%22&hl=pt-PT&sa=X&ei=mbnWUsvDIfTB7Aaw_YD4Dw&redir_esc=y; the quotation in French does not, however, appear to be original, and is probably a relatively modern invention, only quoted in recent (21st century) published works, which attribute it to "Voltaire" without citing any source.
Attributed

This attribution to Voltaire appears in Strauss' Familiar Medical Quotations (1968), p. 394, and in publications as early as 1956 http://books.google.pt/books?id=lCtCAQAAIAAJ&q=%22Doctors+are+men+who+prescribe+medicine+of%22&dq=%22Doctors+are+men+who+prescribe+medicine+of%22&hl=pt-PT&sa=X&ei=mbnWUsvDIfTB7Aaw_YD4Dw&redir_esc=y; the quotation in French does not, however, appear to be original, and is probably a relatively modern invention, only quoted in recent (21st century) published works, which attribute it to "Voltaire" without citing any source.
Original: (fr) Les médecins administrent des médicaments dont ils savent très peu, à des malades dont ils savent moins, pour guérir des maladies dont ils ne savent rien.

This has been reprinted many times with slight variations on the wording; it is part of a much larger quote directly from Edison published in 1903:
:Nineteen hundred and three will bring great advances in surgery, in the study of bacteria, in the knowledge of the cause and prevention of disease. Medicine is played out. Every new discovery of bacteria shows us all the more convincingly that we have been wrong and that the million tons of stuff we have taken was all useless.
The doctor of the future will give no medicine, but will instruct his patient in the care of the human frame, in diet and in the cause and prevention of disease.
They may even discover the germ of old age. I don't predict it, but it might be by the sacrifice of animal life human life could be prolonged.
Surgery, diet, antiseptics — these three are the vital things of the future in preserving the health of humanity. There were never so many able, active minds at work on the problems of diseases as now, and all their discoveries are tending to the simple truth — that you can't improve on nature.
:* As quoted in "Wizard Edison" in The Newark Advocate (2 January 1903), p. 1 according to research by Barbara and David P. Mikkelson at snopes.com http://www.snopes.com/quotes/edison.asp.
1900s