Aristotle (384 BC - 322 BC) was a Greek philosopher, a student of Plato, and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology and zoology.
Together with Plato and Socrates, Aristotle is one of the most important founding figures in Western philosophy. He was the first to create a comprehensive system of Western philosophy, encompassing morality and aesthetics, logic and science, politics and metaphysics. Aristotle's views on the physical sciences profoundly shaped medieval scholarship, and their influence extended well into the Renaissance, although they were ultimately replaced by modern physics. In the biological sciences, some of his observations were only confirmed to be accurate in the nineteenth century. His works contain the earliest known formal study of logic, which were incorporated in the late nineteenth century into modern formal logic. In metaphysics, Aristotelianism had a profound influence on philosophical and theological thinking in the Islamic and Jewish traditions in the Middle Ages, and it continues to influence Christian theology, especially Eastern Orthodox theology, and the scholastic tradition of the Roman Catholic Church. All aspects of Aristotle's philosophy continue to be the object of active academic study today.
Though Aristotle wrote many elegant treatises and dialogues (Cicero described his literary style as "a river of gold"), it is thought that the majority of his writings are now lost and only about one third of the original works have survived.
At about the age of eighteen, Aristotle went to Athens to continue his education at Plato's Academy. Aristotle remained at the academy for nearly twenty years, not leaving until after Plato's death in 347 BC. He not only studied almost every subject possible at the time, but made significant contributions to most of them. In physical science, Aristotle studied anatomy, astronomy, economics, embryology, geography, geology, meteorology, physics and zoology. In philosophy, he wrote on aesthetics, ethics, government, metaphysics, politics, psychology, rhetoric and theology. He also studied education, foreign customs, literature and poetry.
Like his teacher Plato, Aristotle's philosophy aims at the universal. Aristotle, however, found the universal in particular things, which he called the essence of things, while Plato finds that the universal exists apart from particular things, and is related to them as their prototype or exemplar. For Aristotle, philosophic method implies the ascent from the study of particular phenomena to the knowledge of essences, and "form" still refers to the unconditional basis of phenomena but is "instantiated" in a particular substance.
In Aristotle's terminology, "natural philosophy" is a branch of philosophy examining the phenomena of the natural world, and included fields that would be regarded today as physics, biology and other natural sciences. In modern times, the scope of philosophy has become limited to more generic or abstract inquiries, such as ethics and metaphysics, in which logic plays a major role. Today's philosophy tends to exclude empirical study of the natural world by means of the scientific method. In contrast, Aristotle's philosophical endeavors encompassed virtually all facets of intellectual inquiry.
The word metaphysics derives from the fact that Aristotle's musings about divine reality came after ("meta") his lecture notes on his treatise on nature ("physics"). The term is in fact applied to Aristotle's own teacher, and Plato's "metaphysics" is understood as Socrates' division of reality into the warring and irreconcilable domains of the material and the spiritual. The theory has been of incalculable influence in the history of Western philosophy and religion.
Since he was perhaps the philosopher most respected by European thinkers during and after the Renaissance, these thinkers often took some of Aristotle's erroneous positions as-given, which held back science in this epoch. However, Aristotle's scientific shortcomings should not mislead one into forgetting his great advances in the many scientific fields. For instance, he founded logic as a formal science and created foundations to biology that were not superseded for two millennia. Moreover, he introduced the fundamental notion that nature is composed of things that change and that studying such changes can provide useful knowledge of underlying constants.
Much more to read at Wikipedia: Aristotle