In his text, On the Advantage & Disadvantage of History for Life, (1874) Nietzsche addressed the task of human existence, what he called, the task of living, in appropriation of history or better say in the relationship between life and historical knowledge. The quest for truth is also part of the task of existing. The values of this quest for the truth, like any other human related task, is intertwined with the nature of human existence. Now the question is what if some knowledge, some truth, proves harmful and deadly? What if in regard to some truth we are faced with this dilemma; know the truth and die or live and continue in error. Nietzsche recognized this dilemma and argued that given the choice between life and knowledge/truth there is no question about which we ought to choose; any knowledge which destroys life destroys itself, for knowledge presupposes life.
From the same text I have somewhat of a long excerpt, which I strongly recommend you to read. What interests me in this text is the complicated position that man has in regard to the truth(s); it is not as simple as the representation of many thinkers before and after Nietzsche on the matter.
"All acting requires forgetting, as not only light but also darkness is required for life by all organisms. A man who wants to feel everything historically would resemble someone forced to refrain from sleeping, or an animal expected to live only from ruminating and ever repeated ruminating. So: it is possible to live with almost no memories, even to live happily as the animal* shows; but without forgetting it is quiet impossible to live at all. Or to say it more simply yet: there is a degree of insomnia, of rumination, of historical sense which injures every living thing and finally destroys it, be it a man, a people, or a culture.
To determine this degree, and through it the limit beyond which the past must be forgotten if it is not to become the gravedigger of the present, one would have to know precisely how great the plastic power of a man, a people or a culture is. I mean the power distinctively to grow out of itself, transforming and assimilating everything past and alien, to heal wounds, replace what is lost and reshape broken forms out of itself. There are men who have this power to so small a degree that they will incurably bleed to death over a single experience, a single pain, frequently over a single delicate injustice, as from a quite a small bleeding laceration. On the other hand there are those who are affected so little by the wildest and most gruesome calamities of life and even by their own malicious acts, that in midst of them or shortly thereafter they achieve a tolerable degree of well being and a kind of clear conscience.* [...] What such a nature* cannot master, is how to forget; it no longer exists, the horizon is closed and whole, and nothing can serve as a reminder that beyond this horizon there remain men, passions, doctrines and purposes. And this is a general law: every living thing can become healthy, strong and fruitful only with in a horizon; if it is incapable of drawing a horizon around itself or, on the other hand, too selfish to restrict its vision to the limits of the horizon drawn by another, it will wither away feebly or over-hastily to its early demise.
Cheerfulness, clear conscious, the carefree deed, faith in the future, all this depends, in the case of an individual as well as of people, on there being a line which distinguishes what is clear and in full view from the dark and unilluminable; it depends on one's being able to forget at the right time as well as to remember at the right time; on discerning with strong instinctual feelings when there is need to experience historically and when unhistorically. Precisely this is the position the reader is invited to consider: the unhistorical and historical are equally necessary for the health of an individual, a people, and a culture."*
* Light and dark being a metaphor for remembering and forgetting.
*Animals not having the sense of history (past, and future.)
* Please note that in this argument Nietzsche is not praising "a kind of clear conscience." Nietzsche has been misinterpreted many times by his future readers, and some times these misinterpretations resulted in horrible events.
*Those who are affected little by wildest calamities of life.
*Translated by Peter Preuss, 1980.
