Nietzsche opens by questioning the will to truth that makes us such inquisitive creatures. Of all the questioning this will excites in us, we rarely question the value of truth itself.
Nietzsche confronts what he calls the "faith in opposite values." This is the belief that the world can be divided into opposites, starting with the opposition of truth and falsehood. Nietzsche suggests that perhaps the relationship between so-called "opposites" is far more complex. Often, our "truths" are born from our prejudices, from our will to deceive; they are born from our falsehoods.
For instance, conscious thinking is usually contrasted with instinct, but Nietzsche argues that most conscious thinking tends to be informed precisely by instinct. Instinctively, we value truth over falsehood, but perhaps falsehood can be a valuable--even indispensable--condition for life. While philosophers generally would like to proclaim their objectivity and disinterestedness, their instincts and prejudices are usually what inform them. At bottom, we find a bunch of old prejudices called "truths" and a whole system of philosophy built up after the fact to justify these "truths." Nietzsche believes that every philosophy is, essentially, the confession of a philosopher, and it gives us more of an insight into that philosopher's character than anything else.
To elaborate on this point, Nietzsche examines a number of different philosophers, beginning with the Stoics. These philosophers who urged us to live "according to nature" were not trying to re- create us in the image of nature (which Nietzsche argues is absurd) but were trying rather to re-create nature in the image they desired. Philosophy, "the most spiritual will to power," says Nietzsche, "always creates the world in its own image; it cannot do otherwise." This will to power, according to Nietzsche, is our cardinal instinct, more fundamental even than the instinct of self-preservation.
Nietzsche also dissects anti-realism, Kantianism, and materialistic atomism. He argues that ##Kant## never gives anything more than circular reasons for believing that there is a faculty capable of synthetic a priori judgments. Nonetheless, we need to believe in synthetic a priori judgments and will believe in such a faculty even though we don't actually have it.
Another prejudice of philosophers is the belief in "immediate certainties," the most famous of which is ##Descartes##' assertion that he cannot possibly doubt that he is thinking. This certainty only reflects a lack of reflection on what is meant by "I think." Why am I so certain that it is "I" that thinks? That I am the cause of the thinking? Doesn't a thought come to me, isn't it the thought that thinks? And how can I know, without further assumptions or certainties, that I am thinking, and not willing or feeling or something else?