Aristotle of Stagirus (384 BCE – 322BCE) was born in 384 BCE in Stagirus, a Greek colony and seaport on the coast of Thrase. His father, Nichomachus, was court physician to King Amyntas of Macedonia. The Macedonian Court would have a considerable influence on his life. Aristotle's father died while he was a child, and it was his guardian, Proxenus, who sent him to Athens at age 17. At the time, Athens was considered the intellectual center of the world, and here he joined the Academy to study under Plato. Aristotle attended Plato's lectures for twenty years, eventually lecturing himself, particularly on the subject of rhetoric. When Plato died in 347, Aristotle's ability and position in the Academy might have seen him take on leadership there, but the differences in his teachings from Plato's made this impossible, and it was Plato's nephew Seusippus who was chosen to take on leadership of the academy. Aristotle moved to the court of his friend Hermeas, the ruler of Atarneus and Assos in Mysea. It was during his three-year stay here that he married Pythias, Hermeas' niece. In 345 Hermeas was taken over by the Persians, and Aristotle moved to the island of Lesbos, at Mytilene. It is here that he met Theophrastus, his collaborator in scientific endeavors, and later his successor as head of the Lyceum in Athens. The observations of flora and fauna made by Aristotle in his writings on biology were made during his time in Mysea and Lesbos.

In 343 Aristotle was invited back to Macedonia by the King to tutor the young Alexander. The differing ambitions of the two friends (teacher and student), made for an intense scholarly relationship. Aristotle was working on establishing a new philosophical world-view with Greece at its center, however Alexander was preparing to conquer an empire far exceeding the boundaries of the Greek world. Aristotle supported a nationalistic strategy, which would protect Greek culture from the "barbarians," whereas Alexander eventually supported the inclusion of non-Greek culture into his policies to ease his rule of the outer provinces.

When Alexander succeeded King Philip and launched his first series of campaigns, extending his rule to India and Egypt, Aristotle returned to Athens for the first time since Plato's death. Platonism had become the dominant philosophy of Athens, and the Platonic school was running successfully under the leadership of Xenocrates. Aristotle decided to establish his own school at the Lyceum. For the next thirteen years he concentrated on teaching and writing the philosophical treatises, which would become his published works. In the morning at the Lyceum Aristotle would facilitate detailed discussions for his more advanced students, and in the afternoon he would lecture on popular topics of discourse for a general body of philosophers and students. His followers became known as "peripatetics," meaning "to walk about," referring to Aristotle's habit of walking back and forth while relating his lectures.

In 323 Alexander the Great was killed while on campaign in the East, and anti-Macedonian sentiment reigned in Athens. In an attempt to free Greek city-states from Macedonian rule, the Athenian Assembly declared war against Antiipon, Alexander's successor. In a repetition of his experience in 347, Aristotle was considered pro-Macedonian and therefore anti-Athenian, and he was charged with "impiety." This is the same charge that had led to Socrates' execution in 399. To avoid Socrates' end, Aristotle went into voluntary exile to the city of Chalcis. Here he lived with his second wife, Herpyllis, who was also the mother of his son, Nichomachus. In 322 BCE, at age 63, Aristotle died of a digestive ailment.

Aristotle wrote treatises covering a vast range of philosophical thought, from biology, physics, logic, science, and metaphysics to ethics, morality, aesthetics, and politics. He developed a non-Platonic theory of form, produced a system of deductive reasoning for both universal and existential statements, and theorized on the cosmos, life, matter and mind, and the "good life." There are 150 philosophical treatises thought to have been written by Aristotle, 30 of which survive today. It is not certain how many of these treatises are actually unpolished lecture notes, and it is thought that some may be the work of students from the Lyceum instead of written by the hand of Aristotle.

Theophrastus, Aristotle's friend and collaborator from Lesbos who inherited leadership of the school at Lyceum, is reported to have taken care of Aristotle's texts. This collection was in turn passed on to Theophrastus' pupil Neleus, and from him to his heirs, who protected the writings in a vault where they suffered from dampness and pests. The vault was discovered in 100 BCE by a book lover named Apellicon, who brought the rotting texts to Athens. They were moved to Rome in 86 BCE when Athens was captured by Sulla, and in Rome they were published in a new edition due to growing interest of local scholars. Aristotle's work and philosophy in general enjoyed a renaissance at this time, and it is this collection of writings that forms the basis of our studies of Aristotle today.

Aristotle's work enjoyed another rediscovery in the later Middle Ages, when it was studied by medieval scholars. He was called "Ille Philosophus" (the philosopher) by his medieval followers, and his work was discussed as the eternal truth (barring any of his writings that may have contradicted the Bible). Known as Scholasticism, Aristotelian philosophy reconciled with Christian doctrine became the official philosophy of the Roman Catholic Church. Scientific discoveries of the Middle Ages and Renaissance paid heed to Scholasticism, or suffered harsh criticism.

Plato theorized that ultimate reality is only knowable through reason and reflection, and he located it in ideas or eternal forms. Aristotle differed from his teacher, theorizing that ultimate reality is knowable through experience, residing in physical objects, and his writings were often based on first-hand observation. Aristotle's objects (which include organisms) are comprised of form and matter, or their reality and their potential. For example, a block of wood (matter) has the potential to assume whatever form a carpenter chooses to give it, and a seed has the potential to grow into a living tree. Aristotle identifies the form in living creatures with the soul, and describes a hierarchy of souls where plants have the lowest kind, animals a higher kind because of their ability to feel, and humans the highest because of their ability to reason and rationalize.

Change was cyclical to Aristotle, like the cycle of water through evaporation, rain, rivers, oceans and deserts. He imagined an eternal universe without beginning or end, and this is the most basic difference between his work and that of both medieval and modern thinkers. He believed that the overall conditions of the world would never change.