Aristotle (384–322 B.C.), son of a physician, student in Plato's Academy for twenty years, teacher of Alexander the Great, rivals Plato as the most important philosopher in the Western tradition. He started the second great university, the Lyceum, nicknamed “Peripatetic” because he and the students frequently walked in the courtyard while conversing. Reputed to have written 400 books, he was an encyclopedic scholar and writer. The Father of Logic, a great biologist and physicist, writer of the second great work on ethics (accepting the Republicas the first) and great works on political philosophy, he made significant contributions to every area of philosophy. He stammered. He wrote his treatise Ethics (included below) to his son Nicomachus. After Alexander's death, finding himself unpopular in Athens, charged with the crime of impiety, he fled the city, “lest the Athenians sin twice against philosophy” (the first time was in executing Socrates).
Both Plato and Aristotle sought knowledge as a good in its own right. Both were teleologistsin that they believed in a universal goal for all things, the Good or the final cause. In this they differ from modern science (nature knows no purposes), especially Darwinian evolution where chance or random selection and survival of the fittest replace rational order. Aristotle believed in teleology because of the regularity in generation, astronomy, and physical behavior. Nature has unconscious purposes.
Whereas Plato was mystical, an idealist, loved abstract ideas, and scorned the physical, empirical, and sensual, Aristotle was more commonsensical, loved facts, physics, biology, and scientific observation. Plato offers us grand allegories and myths to convey a comprehensive world view, whereas Aristotle sought to remove myth and symbol from philosophy, substituting rational explanation and logical precision.
Aristotle's main disagreement with Plato is over the theory of the Forms. He criticizes it from various points of view—as a logician he offers a different analysis of predication; as an ontologist and epistemologist he argues that it is the concrete particular, not the universal or form, that is substance in the primary sense, and that provides the starting point in our investigation of form. Third, as a physicist he points out that the separate and transcendent forms are useless in accounting for change and coming into existence. Plato's Forms belong to the world of unchanging being, but the study of change involves the investigator of forms in the changing objects of the world of becoming. Aristotle shifted the focus from the study of pure, immutable being to physics, the study of Nature (and natural change).