 This knowledge, which represents and accounts for reality and the way things are, also corresponds, Avicenna maintains, with what is found in books, i.e. with philosophy, or more specifically, with the philosophical sciences as classified and taught in the Aristotelian tradition. However, the identity between absolute knowledge, in the form of the intelligibles contained in the intellects of the celestial spheres, and philosophy, as recorded in the Aristotelian tradition, is not complete. Though Aristotelianism is the philosophical tradition most worthy of adherence, Avicenna says, it is nevertheless not perfect, and it is the task of philosophers to correct and amplify it through the acquisition of further intelligibles by syllogistic processes. It is this understanding that enabled Avicenna to have a progressive view of the history of philosophy and set the framework for his philosophical project. For although the knowledge to be acquired, in itself and on the transcendent plane of the eternal celestial intellects, is a closed system and hence static, on a human level and in history it is evolutionary. Each philosopher, through his own syllogistic reasoning and ability to hit correctly upon the middle terms, modifies and completes the work of his predecessors, and reaches a level of knowledge that is an ever closer approximation of the intelligible world, of the intelligibles as contained in the intellects of the spheres, and hence of truth itself. Avicenna was conscious of having attained a new level in the pursuit of philosophical truth and its verification, but he never claimed to have exhausted it all; in his later works he bemoaned the limitations of human knowledge and urged his readers to continue with the task of improving philosophy and adding to the store of knowledge.