In the Science of Logic Hegel presents his speculative logic, the system of categories through which thought thinks itself. By developing the categories one from the other, thought moves immanently, without losing its unity. The justification of philosophy can only be reached within its own interior, and by reaching the Absolute Idea, the final category, with which thought truly expresses itself, it would reach the circularity capable of providing this self-justification. But such circularity does not seem to be enough to deal with the skepticism aroused by the perplexity faced with the diversity of philosophical systems. Hegel then proposed a philosophy of the history of philosophy according to which the truth reached by speculative logic would manifest itself in history, converting it into an argument in favor of, and not against, the truth of his philosophy.