Aristotle (384-322 BCE) - in Greek Ἃριστοτέλης (Aristoteles); English pronunciation AR-is-to-tel, IPA: /'ær ɪ ˌstɒt əl/ - was born in Stagira in northern Greece. His father Nicomachus was physician to the king of Macedonia, and Aristotle probably spent many of his earliest years at the Macedonian court. At the age of 17 he travelled to Athens to study in Plato's Academy, where he remained first as a student and then as a researcher until Plato's death in 347. The next four years were spent at Assos on the coast of Asia Minor and on the island of Lesbos (Mytilene), the site of many of Aristotle's zoological investigations. In 343 King Philip of Macedon invited Aristotle to be tutor to his son, the future Alexander the Great, a post he held until Alexander succeeded his father on the Macedonian throne in 336. Aristotle then returned to Athens and founded his own research institute or private university, the Lyceum. He taught and pursued his own researches in the Lyceum until 323, when the strength of anti-Macedonian feeling in Athens obliged him to move to the island of Euboea, where he died the following year.
Aristotle's earliest works were dialogues in the style of Plato, but of these only fragments have survived. His mature works are in the form of treatises, and many of them probably reflect courses he gave at the Lyceum during the last decade of his life. These treatises deal with an immense range of subjects. Aristotle not only made contributions of the highest value in all the branches of philosophy, but initiated the sciences of biology and zoology, and wrote on political theory and literary criticism.
Although as a young man Aristotle was sympathetic to Plato, his own philosophical outlook stands in sharp contrast to Plato's. Aristotle rejected Plato's Theory of Forms, denying the existence of the timeless and immaterial entities - the Forms - which Plato argued were fundamental to a proper understanding of the world. His own view of the world is more in line with common sense, and is elaborated and explored with great sophistication in his Metaphysics. Of all Aristotle's works, however, the most influential has probably been his Nicomachean Ethics, in which he outlines his account of human happiness or well-being (eudaimonia, εὐδαιμονία), presents his famous Theory of the Mean, and discusses many of the moral virtues such as courage and justice.
As well as his influence on many philosophers in the ancient world - his followers were known as Peripatetics - Aristotle exerted a considerable influence on Islamic science and philosophy and on medieval Christian philosophers, and in particular on Thomas Aquinas (1225-1274 CE).
For some help with the pronunciation of Greek names see Pronunciation of Greek Proper Names.