A major leitmotiv characterizing Nietzsche’s discourse on truth is his insistence on the fact that truth is, by definition, antagonistic or foreign to life.
Truth is not and cannot be a function of survival, since it is more harmful than beneficial to life (although, with its inexorability and hardness, it can also strengthen life).
This stance could be recognized as the origin of Nietzsche’s own oscillation between, on the one hand, the depreciation of truth as an enemy of life and, on the other, a kind of ethical imperative to pursue the truth, advocating (or, at least, valuing) a certain heroism of truth.
One cannot simply “live in truth”—something like “time without truth” is essential to life.
Thus, for example, the great empiricist Locke writes: "He that will not eat till he has demonstration that it will nourish him; he that will not stir till he infallibly knows the business he goes about will succeed, will have little else to do but sit still and perish.”
We go through life relying on all kinds of probabilities and beliefs; we rarely have the certainty of truth at hand, and this is precisely what endows us with a capacity for action.
To say, as Nietzsche does, that “untruth is a condition of life” is indeed a strong, powerful thesis, yet one that aims at exactly the same point: it is not possible to live in truth; truth is not the adequate medium of life.
The two passages quoted above are both, despite their apparent incompatibility, expressions of only one of these two lines of argumentation.
The apparent contradiction between them disappears in the light of a thesis that Nietzsche keeps repeating (in different forms): “so far, the lie has been called truth.”
The first passage is simply an elaboration of this thesis: what Nietzsche calls “untruth,” celebrating its contribution to life, is precisely what has so far been called truth (synthetic a priori judgments, unconditional, logical fictions, mathematics as based on numbers, etc.).
The second passage insists on the fact that what has thus far been called truth can no longer be so called, that truth is elsewhere, and that the first statement of this new truth is precisely that, hitherto, the lie has been called truth (or, in another formulation: "I was the first to discover the truth by being the first to experience lies as lies”).
This is the truth that has “always been forbidden.”
But, even though there is no conceptual contradiction between the two passages (they are both based upon the thesis that “so far,lie has been called truth”), there is a clear contradiction or difference in their orientation: one prizes the (newly recognized) lie as a condition of life, and its flourishing; while the other prizes the courage to pursue the truth at the cost of its danger (to our well-being and life).