Foucault has a distinctly non-realist take on what Nietzsche means here. He distinguishes two uses of truth:
—Truth is not true if it is knowledge, since all knowledge is an illusion. —Truth is not true insofar as it is non-knowledge, since it superim-poses on knowledge or replaces knowledge with a system of error. —Truth is not true when it claims to be knowledge, it is lie.
Which allows us:
a—to lay down as principle that truth cannot be the predicate of itself. Truth is not true. All truth is deployed in the non-true; the truth is non-true. There is no ontology of truth. In the predicative judgment: truth is true, the verb to be has the ontological meaning: truth exists. Nietzsche transforms the skeptical assertion “truth does not exist” into a series of paradoxes deriving from the proposition: truth is not true.
b—to distribute the major categories of the non true truth:
—illusion, that is to say, truth insofar as it is a mode of knowledge; —error, insofar as it is violence done to knowledge (and therefore non-knowledge); —lie, insofar as this non-knowledge claims to dissipate the illusion of all knowledge although it is knowledge.
Starting from here, we can see the Nietzschean task: to think the history of truth without relying on truth. In an element where truth does not exist: this element is appearance. Appearance, this is the element of the non-true within which the truth dawns. And in doing so it redistributes appearance into the cat-egories of illusion, error, and lie. Appearance is the indefinite of truth. Illusion, error, and lie are the differences introduced by truth into the game of appearance. But these differences are not only the effects of truth; they are truth itself. We can also say:
—Truth makes appearance appear as illusion, error, lie.
Or:
—Illusion, error, and lie is the mode of being of truth in the indefinite element of appearance. —Illusion, or the root of truth. —Error, or the system of truth. —Lie, or the operation of truth.
See the texts on truth as error:
“Truth is a sort of error.”
“What in the final instance are man’s truths? They are irrefutable errors.”
On the renunciation of truth:
“The belief that there is no truth , the nihilist belief, is a great relaxation of all the limbs for the champion of knowledge who is constantly struggling with ugly truths.”
A conviction that no epoch has ever had: we do not have the truth. Previously everybody had the truth, even the skeptics. On appearance:
“‘ Appearance ,’ as I understand it, is the true and sole reality of things, that to which all existing predicates are suited … I do not posit ‘appearance’ as the opposite of ‘reality’; I assert rather that appearance is reality, that it is that which is opposed to what transforms reality into an imaginary ‘true world‘.
To summarize: In Aristotle, the will to know derived from the preexistence of knowledge; it was nothing other the delay of knowledge with regard to itself and that is why it was desire, even less than “desire,” it was desire-pleasure. And this was possible only insofar as knowledge (in the most elementary form of sensation) was already related to truth. In Nietzsche, knowledge is an illusory effect of the fraudulent assertion of truth: the will that brings both of them has this double character: (1) of not being will to know but will to power; (2) of founding a relationship of reciprocal cruelty and destruction between knowledge and truth. The will is what says in a double and superimposed voice: I want the truth so much that I do not want to know and I want to know up to that point and that limit that I wish there was no longer any truth. The will to power is the breaking point at which both truth and knowledge come apart and destroy each other.