The book breaks with Nietzsche's previous essay style (as in The Birth of Tragedy). It is a collection of aphorisms, largely concerned with human psychology. He criticizes social Darwinism in it:
Wherever progress is to ensue, deviating natures are of greatest importance. Every progress of the whole must be preceded by a partial weakening. The strongest natures retain the type, the weaker ones help to advance it. Something similar also happens in the individual. There is rarely a degeneration, a truncation, or even a vice or any physical or moral loss without an advantage somewhere else. In a warlike and restless clan, for example, the sicklier man may have occasion to be alone, and may therefore become quieter and wiser; the one-eyed man will have one eye the stronger; the blind man will see deeper inwardly, and certainly hear better. To this extent, the famous theory of the survival of the fittest does not seem to me to be the only viewpoint from which to explain the progress of strengthening of a man or of a race.§224
Nietzsche also distinguishes in this work the obscurantism of the metaphysicians and theologians from the more subtle obscurantism of Kant's critical philosophy and modern philosophical skepticism, claiming that obscurantism is that which obscures existence rather than obscures ideas alone: "The essential element in the black art of obscurantism is not that it wants to darken individual understanding but that it wants to blacken our picture of the world, and darken our idea of existence" (Vol. II, Part 1, 27).