It should go without saying that the process of thought is something of import to all philosophers. After all, all ideas come from a thought; therefore, study of the process by which we construct ideas from thoughts is a necessity. Nietzsche takes a very interesting approach to the human thought process, saying that trial and error is the foundation of cognition. The will of the spirit and the will to knowledge often can come into conflict, as the spirit can be decidedly deceptive or ignorant to knowledge; however, both wills are integral to humankind, shaping the way we view the rest of the world.
Nietzsche likens the construction of thought to the construction of a crystal lattice. In a crystalline structure, new parts of the crystal form on top of the old parts, but their orientation is based upon the parts of the crystal it's attached to, such that the overall structure is uniform. "In our thought, the essential feature is fitting new material into old schemas..., making equal what is new." (WP 499) Everything that we learn, perceive, and create is affected by what it is that we already have learned, perceived and created, such that it is inherently and uniformly ours and ours alone. Just like the crystal, our knowledge base can be expanded upon. The most efficient and basic way of doing this is through the trial and error process. We invent an idea, establish its truth or falsity, and assimilate it into the knowledge base. Error, he believes, is much more valuable to thought than truth is. If we establish something as truth, we could expand upon it, but may not be as motivated to. On the other hand, when we make a mistake, we must ask ourselves why it happened thus, analyzing what it was that led to the error. In short, it spurs on much more thought, about things that maybe are not directly related to our original thought, but nonetheless relevant.
With so much knowledge available in the world, and such a desire to learn (we all make plenty of mistakes, don't we?), there must be some will that can counteract the will to knowledge. Nietzsche terms this the "will of the spirit." (BGE 230) This will allows for four methods of manipulating knowledge, and thus thought: assimilation, ignorance, deception of others, and self-deception. All four of these processes alter knowledge in some way. Through assimilation, we equate present to past, though the present knowledge is not just fitted into our established knowledge base, but is altered to serve whatever our needs are at the moment. While this seems very similar to the will to knowledge, the fact that we permute the knowledge at all makes that truth dubious at best. We use ignorance as a way of putting aside - or defending against - knowledge which may be beyond our spirit's "digestive capacity." (BGE 230) Self-deception occurs on occasion, out of a desire for uncertainty and ambiguity.
Nietzsche discusses the deception of others in detail as half of BGE 230. It seems to Nietzsche that not only do we do it frequently, we need to do it because we receive gratification from doing it. When we deceive others, it is akin to wearing a mask - we hide what is ourselves behind something we've created. At times, we revel in the skill with which we construct these veils; at other times, we enjoy the security they provide to those parts of ourselves that we wish not to reveal. In reality, though, every surface is a cloak of sorts. Our basic insides are hidden by skin, which in turn we willfully hide behind layers of clothing. Our beliefs and ideas are filtered through words, chosen carefully dependant upon the situation.
This will to deceive is countered by the will to knowledge, a will whose curiosity leads him to leave no cloak undisturbed, to know that which was hidden from his initial inquiry regardless of the lengths it was gone to for that knowledge to be hidden or the results of his disturbance. Nietzsche refers to this as "a kind of cruelty of the intellectual conscience and taste", (BGE 230) and believes that any thinker who has peered into his own motives with enough discipline and harsh self-analysis can see this within himself so strongly that no one could make him believe that there is no cruelty within him. (In thinking about this, one must consider why he hides parts of himself, as anyone who wanted to poke around behind his masks would surely be accused of some form of cruelty.) In truth, then, everyone has this "cruel" element to their character if they are actively learning - some just don't see it as such. They deny any cruelty, calling it (in Nietzsche's terms) "extravagant honesty" on the part of humanity. They hide the cruelty behind various terms (love of truth, sacrifice for knowledge, etc.), which is "worthy verbal pomp... [which] belongs to the old mendacious pomp, junk, and gold dust of unconscious human vanity." (BGE 230) However, in the end, it is all part of human nature, and must be recognized as such. This basic human nature can be risen above, though. Nietzsche believes that our ability to overcome human nature is a direct product of the existence of knowledge in the first place - a "strange and insane task," (BGE 230) he notes. As strange and insane as it may seem, what is the utility in knowledge and thought to us, as humans, if we cannot utilize it to improve ourselves? Nietzsche admits that this is one cloak that might prove impossible to lift.
The problem in lifting this cloak lies in the very method that humanity uses in its search for true knowledge - language. "[W]e speak of trees, colors, snow, and flowers; and yet we possess nothing but metaphors for things". (PT - "On Truth and Lies in a Nonmoral Sense" 1) If no two items in this world are exactly equal - and they can't be, or else they would be each other - then any way of referring to them as the same thing is, on some level, a lie. So all we have are these "metaphors for things", the origins of which are not based in reality, but their assimilation into our human perception of the world. "What then is truth? A movable host of metaphors, metonymies, and anthropomorphisms: in short, a sum of human relations which have been poetically and rhetorically intensified, transferred, and embellished, and which, after long usage, seem to a people to be fixed, canonical, and binding. Truths are illusions which we have forgotten are illusions". (PT - "On Truth and Lies in a Nonmoral Sense" 1) These truths are the basis for our "great edifice of concepts"; the blueprint for that structure is none other than ourselves, how we experience the world, how we relate to our world - from our point of view. Is it the proper relation? Not necessarily.
Nietzsche argues that, because each individual part of nature (his example is based on the atomic level, which, for his time, was the smallest unit understood by science) interacts with the rest of nature, everything has some form of sensation. In a universe where everything is sensate, there is so much happening all at once that individual centers of sensation are confused, regardless of size. "Whether larger or smaller, these sensation complexes would be called 'will'." (PT-"The Phil" 96) The combination of the inborn qualities of sensation and reflex (which is preserved by memory) leads us to the concept of causality, and "[s]pace and time depend upon the sensation of causality." (PT-"The Phil" 97) What gives us consciousness is this knowledge of causality - we construct an understanding of the world, within and without, through causal relationships. Our memory is based more on the existance of sensation than as a tool to hold our store of reflexive actions; for example, being part of nature, trees are sensate, they have memory (rings) yet no reflexive actions. The fact that there is a certain set of laws that nature is ruled by is considered proof that everything has the two basic qualities of sensation and memory. When two seperate interactions of identical systems are constructed, the effects of those interactions are the same. Nietzsche says that the effect of an initial reaction are based on the "pleasure and displeasure" of the reactants, and that subsequent reactions are governed by this "pleasure principle" which, through memory and sensation, becomes a learned behavior - a natural law.
Nietzsche takes a stand against many other philosophers (notably, Plato) that "qualities" are not native to the objects themselves (i.e., Plato's "forms"), but to those who bestow such qualities onto the world. We create the measures by which we determine what qualities things do or do not possess. In fact, no knowledge at all could exist without the ability to measure one thing against another. We compare and contrast what is new based on what we already understand, and build it into our knowledge base thusly, as per the crystal example in WP 544. If all our human knowledge is based on sensation, then all human senses - and, it follows, parts of our knowledge base - are identical. Because we share such a base level of knowledge, we over-simplify and over-generalize all of nature by anthropomorphizing it, creating a "mirror" through which we see the world. The nature of science, Nietzsche claims, is to make this mirror clearer by removing those human traits we place upon things with the "natural law" it actually follows. It thus follows that our entire thought process, the will to knowledge, as it were, is only as strong as our desire to remove any veils that we wish to remove.
Ed. note: All boldfaced book references are listed as (BOOK #aphorism). Book codes are: BGE = Beyond Good and Evil; PT = Philosophy and Truth - "The Phil" = "The Philosopher," a section in PT; WP = The Will to Power.