In The Gay Knowledge, Nietzsche proposes a reflection on health, and more particularly the health of the soul. Nietzsche does not imagine a reality separate from the body, which would be the soul. Nietzsche himself was sick all his life.
What he designates by soul is a fundamental stratum, of physiology, which is the expression of a state of the body. “I am all body, and nothing else” as Zarathustra says.
120. Health of the soul. The famous formula of moral medicine (of which Ariston of Chios is the author): “Virtue is the health of the soul” should, if it is to be used, at least be transformed thus: “Your virtue is the health of your soul.” For in itself there is no health, and all attempts to give this name to a thing have failed miserably. It is important to know your goal, your horizon, your strengths, your impulses, your mistakes and above all the ideal and the ghosts of your soul in order to determine what health means, even for your body. So there are innumerable healths of the body; and the more we allow the particular and incomparable individual to raise his head, the more we will unlearn the dogma of “the equality of men”, the more it will be necessary for our doctors to lose the notion of a normal health, of a normal diet, of the normal course of the disease. Only then will it be time to reflect on the health and illness of the soul and to put the particular virtue of each one in this health: it is true that the health of the soul could resemble in one the opposite of the health in the other. And finally the great question would remain open: to know if we can do without illness, even for the development of our virtue, and if particularly our thirst for knowledge and self-knowledge does not need the sick soul as much as the healthy one: in a word if the only will of health is not a prejudice, a cowardice, and perhaps a remainder of the most subtle barbarism and of the retrograde spirit.
Paragraph 120, Nietzsche, The Gay Knowledge (1882)
Nietzsche redefines the philosopher as the medicine of culture. It is no longer the problematic of the search for truth that guides philosophy.
The philosopher is a medical philosopher rather than a scholarly doctor. The big question is to know if we can do without the disease. The question is: are there sick modes of thought, which reflect a deviation from what would be health? Nietzsche answers in the affirmative: there are in a certain way diseases of the soul, symptoms of dysfunctions in the body organization. For example, the proliferation of pessimistic judgments. It is therefore necessary to think dynamically between health and illness. Pessimism is an obstacle to overcome.
” That which does not kill me strengthens me “, explains Nietzche in The Twilight of the Idols published in 1888.