Nietzsche's critique of truth and knowledge in this chapter rests largely on the claim that anything that is made understandable to the majority of people has necessarily been distorted and simplified. Truth and knowledge are thus artificial certainties that people can fall back upon. As Nietzsche suggests in the previous chapter, our "truths" are founded on a bedrock of prejudice.
Because the majority of people remain tied to assumptions and prejudices, they tend to misunderstand truly deep thoughts. We can only understand things on a level that our intellect is capable of handling, and we tend to simplify and caricature ideas that are above us. Thus, Nietzsche suggests, the free spirit must appear "masked" to the masses: people cannot understand such freedom of spirit and so interpret it as something else entirely. This point is particularly apt for Nietzsche, whose writings have been so misunderstood and misinterpreted--notably by the Nazis, who forced a reading of Nietzsche that was quite contrary to his intentions. Nietzsche aims to re-evaluate so many of our assumptions that he is prone to being misinterpreted. Karl Jaspers gives us a clue as to how to read Nietzsche when he says that we should be nowhere satisfied until we have "also found the contradiction."
"Free spirits" are so called because they do not allow themselves to be tied down to any of the certainties or "truths" that are based on prejudice. They engage in a radical skepticism that drives them to question everything. We get a good sense of what this skepticism might entail in Nietzsche's discussion of an "extra-moral" worldview. Our current morality is based on origins and intentions, so that we say a certain action is good or bad depending on the spirit in which it was performed. Nietzsche sees a simplification of the facts in the way this position assumes that our intentions are simple and transparent. Quite to the contrary, he suggests that our outward intentions are a mere surface that covers up a great deal of unconscious motivation. (For instance, one person's kindness to another might be motivated by an unconscious desire on the first person's part to make herself feel superior to the other.)
Late in the chapter, Nietzsche asserts that the new breed of free-spirited philosophers will be "attempters" (or "experimenters," depending on the translation). This title may be meant as a contrast to Nietzsche's earlier labeling of philosophy-to-date as dogmatism. While previous philosophers have built up complex systems meant to justify underlying prejudices, these "attempters" will be remarkable for their flexibility and their careful evasion of all prejudice. They will constantly be juggling new ideas, never discarding them for their unattractiveness, but always greeting them with an open mind. We find Nietzsche giving an example of this experimentalism with his discussion of the will to power.
Nietzsche suggests that if we can identify one efficient cause that can explain all phenomena, we are better off than if we need to rely on many different causes. Nietzsche suspects that the will to power can serve as this one efficient cause, and he suggests that we test this hypothesis experimentally. He believes that all human behavior is dictated by this will, so that, for instance, thought is not an ideally rational and disinterested activity, but is rather a struggle between different drives within the thinker. That I think one thing rather than another is merely a sign that one drive is dominant over another inside me. If this will to power also governs our drive for reproduction and nutrition, we could see it as motivating the whole cycle of life on this planet. Further, he suggests that the will to power isn't present only in living things, but can also be found in dead matter. Rocks and water simply lack the organization and cohesion of a human body, and so lack a focused will to power, but even there the will to power is operating.
Nietzsche is far from careful or precise in what he means by all this, but his discussion of the will to power is only meant to show how his "experimental method" could be carried out: this is not meant to be an instance of it. Of course, Nietzsche falls into the frustrating habit of most philosophers of suggesting that we can work out the details but never bothering to do the detail work himself. Instead, he remains on the level of generalities, a level that is always more prone to error. While Nietzsche's "experiment" may rest upon a bold and ingenious exercise of creativity, it lacks the rigor and detail that the experimental method of science calls for.