The system of thought of the Greek philosopher Aristotle (384-322 B.C.) and his followers. Aristotle divided human knowledge into three categories: the theoretical, conceived with the truth for its own sake; the practical, directed to the guidance of conduct; and the productive, to be used in the cultivation of the arts. This division has ever since affected what may be called Christian Aristotelianism, through Thomas Aquinas (1225-74), who built on Aristotle an impressive philosophy of the Christian faith. On this level, Aristotle's main contributions include: 1. a clear analysis of the foundations of human reasoning, through induction and deduction; 2. the theory of the four causes, material, formal, efficient, and final; 3. hylomorphism, or the theory of matter and form; 4. the principle of teleology that every being has a purpose; 5. the postulate that every living being has a principle of life, which in man is the rational soul; 6. the proof of an eternal unmoved Mover, to explain the existence of a world that is changeable, and the identification of this eternal unmoved First Mover with the god or gods of popular religion; 7. the conclusion that this First Cause is pure thought, who is pure actuality, the only being with no extension, who is the supreme object of all knowledge and the ultimate object of all desire; 8. the definition of the ethical good as that which corresponds to man as rational being and consists in the subordination of the senses to reason and in the exercise of reason to search for and contemplate the truth.