Thought characterizes Homo. "Thinking" is a repeated word, present in any conversation or writing. But is there a detailed definition of what it is strictly? It is unquestionable that it is a mental faculty, that is, a psychic faculty. But then, is it a sense?, is it a feeling?, is it a knowledge?, is it an intuition?, ... No, none of them on its own. Thought is an intimate combination of many other simpler faculties, that is, a clear "complexation" of them. Consequently, it must have a structure based on its generating components. But, has anyone heard which faculties compose it and how they progressively structure it? The reference RAE dictionary for Spanish-Castillian says: "Faculty or capacity to think" which is a tautology; as well as "Action and effect of Thinking", which is a serious polysemy. They do not improve at all in psychology dictionaries, which often solve it with a potpourri of synonyms. And if we search for their structure, proposals like "Concept, judgment, reasoning and demonstration" or similar, there are no structuring. Some even perceive thinking as a vice to be eradicated. It is also common and logical, the question: "What is it that science still does not know?", about which even classifications have been developed, the top 25 problems, the top 125, ... But, curiously, always forgetting the most important thing: "how can man know the many things that we already know?", but neither is asked nor is it known. The "Representation of knowledge" is a detailed description of the psychic processes that allow human knowledge, but surprisingly, it has never been described before! We know everything and a lot, except what allows us to know all this. In my opinion, it is the biggest "black hole" of science. In "The Representation of Knowledge..." (see [A]) it was clear, that even with two searches with "AI" asking: "How do we know?" or "What faculties allow human knowledge?", the ambiguous answers confirm that it is totally unknown to science, a "black hole". And knowledge is undoubtedly at the core of thought, as will be seen. I do not think that 71 pages are many to answer clearly and definitively what thought is and how it is structured, and how knowledge is acquired. Nor do 87 pages to do the same with "What is consciousness(-raising)?" [1], both faculties that have eluded science for centuries. They are also related, which is why I wrote these two articles that explain them in the same year, 2007. This article presents the 4 structural levels of thought, and how each one progressively results from the previous one. 0) A previous level or "level zero" (the "Sensitive Identification", auditory and tactile from the last months of the fetus, and visual from the first months of the baby), 1) a first level of the sensitive or simple Conceptuation from two years of age, 2) a second level of the virtual or composite Conceptuation, 3) the level of strict or relational Knowledge from six years of age, and 4) a fourth level of the Method, from which applications such as Reasoning and Logic are derived. There is a close relationship between this article with the logic from FREGE and GÖDEL. And in addition, I have been writing lately the recent articles of "The knowledge of children at 2 years old" [C], "... at 3 years old" [D], "... at 6 years old", and "... at 10 years old", even more detail the central part of this structure, which is how knowledge progresses with age. "Knowledge" but in its strictest interpretation, the most understanding and relational one, not in synonyms such as "Acquaintance", "Factual or historical knowledge", ... And at the same time it clarifies what other related concepts such as "Abstraction", "Idea", "Understanding", ... The article describes the cognitive evolution of the child, and we have all been there before. It is like the intellectual biography of any person. The reader will be among the first to know what knowledge is and the thought that encompasses it. In addition, intrinsic semantics is introduced, a new discipline that intersects mathematics and psychology, without which it is not possible to do science of knowledge in a serious way. Finally, we say that thought should be free, but, ¿is it true that we can think freely, as we want? Or, seeing the antagonisms that exist, ¿perhaps we can only do so within certain limits, those that condition us by the feelings of our unconscious, intimately related to thought?