In the opening two parts of the book, Nietzsche discusses in turn the philosophers of the past, whom he accuses of a blind dogmatism plagued by moral prejudice masquerading as a search for objective truth; and the "free spirits", like himself, who are to replace them.
He casts doubt on the project of past philosophy by asking why we should want the "truth" rather than recognizing untruth "as a condition of life." He offers an entirely psychological explanation of every past philosophy: each has been an "involuntary and unconscious memoir" on the part of its author (§6) and exists to justify his moral prejudices, which he solemnly baptizes as "truths".
In one passage (§34), Nietzsche writes that "from every point of view the erroneousness of the world in which we believe we live is the surest and firmest thing we can get our eyes on". Philosophers are wrong to rail violently against the risk of being deceived. "It is no more than a moral prejudice that truth is worth more than appearance". Life is nothing without appearances; it appears to Nietzsche that it follows from this that the abolition of appearances would imply the abolition of "truth" as well. Nietzsche asks the question, "what compels us to assume there exists any essential antithesis between 'true' and 'false'?"
Nietzsche singles out the Stoic precept of "living according to nature" (§9) as showing how philosophy "creates the world in its own image" by trying to regiment nature "according to the Stoa". But nature, as something uncontrollable and "prodigal beyond measure", cannot be tyrannized over in the way Stoics tyrannize over themselves. Further, there are forceful attacks on several individual philosophers. Descartes' cogito presupposes that there is an I, that there is such an activity as thinking, and that I know what thinking is (§16). Spinoza masks his "personal timidity and vulnerability" by hiding behind his geometrical method (§5), and inconsistently makes self-preservation a fundamental drive while rejecting teleology (§13). Kant, "the great Chinaman of Königsberg" (§210), reverts to the prejudice of an old moralist with his categorical imperative, the dialectical grounding of which is a mere smokescreen (§5). His "faculty" to explain the possibility of synthetic a priori judgements is likened to the explanation of the narcotic quality of opium in terms of a "sleepy faculty" in Molière's comedy Le Malade imaginaire. Schopenhauer is mistaken in thinking that the nature of the will is self-evident (§19), which is in fact a highly complex instrument of control over those who must obey, not transparent to those who command.
"Free spirits", by contrast to the philosophers of the past, are "investigators to the point of cruelty, with rash fingers for the ungraspable, with teeth and stomach for the most indigestible" (§44). Nietzsche warns against those who would suffer for the sake of truth, and exhorts his readers to shun these indignant sufferers for truth and lend their ears instead to "cynics" — those who "speak 'badly' of man — but do not speak ill of him" (§26).
There are kinds of fearless scholars who are truly independent of prejudice (§6), but these "philosophical labourers and men of science in general" should not be confused with philosophers, who are "commanders and law-givers" (§211).
Nietzsche also subjects physics to critique. "Nature's conformity to law" is merely one interpretation of the phenomena which natural science observes; Nietzsche suggests that the same phenomena could equally be interpreted as demonstrating "the tyrannically ruthless and inexorable enforcement of power-demands" (§22). Nietzsche appears to espouse a strong brand of scientific anti-realism when he asserts that "It is we alone who have fabricated causes, succession, reciprocity, relativity, compulsion, number, law, freedom, motive, purpose" (§21).