In two passages from Twilight of the Idols, Nietzsche makes the argument that negative valuations of life or valuations of life in general could be treated only as symptoms. In other words, Nietzsche here seemingly refuses to engage rationally the arguments that have been proposed in favor of such valuations and proceeds to view them from his typical genealogical perspective. In this article, I show that this initial impression is misleading and that it is possible to reconstruct from these passages in Twilight of the Idols an argument that justifies Nietzsche’s genealogical view of such valuations. According to this argument, it is precisely because the value of life cannot possibly be justifiably estimated that the genealogical approach is warranted. I lay out the argument in detail, discuss some of its premises, and point the way to a more general conclusion.