Nietzsche writes that ‘truths are illusions which we have forgotten are illusions’ in his work On Truth and Lie in an Extra-Moral Sense. In this work, he reasons that human intellect is arbitrary and aimless, not serving a particular beyond-human goal, and is accompanied with pride. This pride in practicing human intellect is deceiving man. Our value for knowing, the drive for truth, immerses us into illusions, which we perfectly accept. He states: “(. . . ) man permits himself to be deceived in his dreams every night of life. His moral sentiment does not even make an attempt to prevent this (. . . )”.

According to Nietzsche, use of language can illustrate how truths relate to illusions. If we try to express reality, we do so by forgetting what ‘words’ are; we give words too much credit. Nietzsche writes: “What is a word? It is the copy in sound of a nerve stimulus. But the further inference from the nerve stimulus to a cause outside of us is already the result of a false and unjustifiable application of the principle of sufficient reason”. Words fail to access the entirety of things of which we talk; only the entirety could be the truth. Consequently, every word becomes a concept, not serving to remind us of its original use (i.e., the attempt to capture the thing from a particular perspective), but rather to serve as a fit for all perspectives. He summarizes that "every concept arises from the equation of unequal things” and exemplifies that "just as it is certain that one leaf is never totally the same as another, so it is certain that the concept ‘leaf’ is formed by arbitrarily discarding these individual differences and by forgetting the distinguishing aspects”.

Truths have become merely a sum of human relations (concepts) which have been poetically and rhetorically intensified, clarified, and which, after long usage, now seem to be anchored and unalterable. Nietzsche therefore concludes: "truths are illusions which we have forgotten are illusions".

Nietzsche writes that this indifference or ignorance to truths is fed by living with the herd. Because our truths are illusions, we do not speak the truth, and therefore lie. In a moral and social sense, it is not lying itself (i.e. the argued deceiving character), but the harmful consequences of lying for which one is imputed and socially excluded. Thus, the illusionary truth serves pleasant, life-preserving consequences and becomes morally justified. Therefore, to be truthful means to employ the prevalent metaphor. This is the duty to lie, the duty which society imposes on truth, and accounts for the title of his work.