Aristotle, the son of Nicomachus, physician at the court of Mayntas II of Macedon, was born in Chalcis and moved to Athens in 367 bc, where he was a member of the academy until Plato's death in 347. For the next 12 years he worked in Assos in Asia Minor, Mytilene on Lesbos, and, from 342 until 335, in Macedon as the tutor of the young Alexander the Great. Unfortunately little is known of this legendary relationship apart from the fact that Alexander took with him on his campaigns a copy of Homer's Iliad annotated by Aristotle. Also, Plutarch quotes a letter from Alexander rebuking his former tutor for publishing his Metaphysics and revealing to all what had been privately and, he assumed, exclusively taught to him. Following Alexander's accession to the throne of Macedon in 335 Aristotle returned to Athens to found his own school, the Lyceum. When, however, Athens, with little cause to love the power of Macedon, heard of the death of Alexander (323) they turned against Aristotle and accused him, as they had Socrates earlier in the century, of impiety. To prevent Athens from “sinning twice against philosophy” he moved to Chalcis where he died the following year.

Aristotle not only developed an original and systematic philosophy but applied it in a deliberate manner to most areas of the knowledge of his day. The resulting treatises on such subjects as physics, cosmology, embryology, and mineralogy acquired a considerable authority, becoming for medieval scholars if not the last word on any subject then invariably the first. Aristotelian science was not overthrown until the great scientific revolution of the 16th and 17th centuries.

In cosmology Aristotle basically accepted the scheme in which the Earth was at the center of the universe with the planets and fixed stars moving around it with uniform speed in perfectly circular orbits. (He also believed, on empirical grounds, that the Earth was round.) But Aristotle was not content simply to construct models of the universe and faced the problem of how to account for the various forms of motion. He began by accepting that matter was composed of the four elements of Empedocles – earth, water, fire, and air. Left to themselves the elements would either fall freely, like earth and water, or rise naturally like air and fire. This for Aristotle was natural motion, self-explanatory and consisting simply of bodies freely falling or rising to their natural place in the universe. For a stone to fall to the ground no one had to push or pull it but merely to remove all constraints for it to fall in a straight line to the Earth.

But the heavenly bodies do not move up or down in straight lines. Therefore, Aristotle concluded, they must consist of a fifth element, aether (or quinta essentia to the medieval schoolmen) whose natural motion was circular. Thus, in the Aristotelian universe different bodies obey different laws; celestial and terrestrial bodies move differently because the laws of motion are different in the heavens from those operating below the Moon. Nor was this the only distinction. For Aristotle the heavens were, with their supposed regularity, incorruptible, without change or decay; such processes were only too apparent on the Earth.

Aristotle also produced a number of volumes on biological problems. In particular his De partibus animalium (On the Parts of Animals) and his De generatione animalium (On the Generation of Animals) show a detailed knowledge of the fauna of the Mediterranean world and a concern to understand their anatomy and physiology. Over 500 species of animal are referred to by Aristotle. He was also a keen observer and had obviously made empirical investigations on the development of the chick embryo for example, noting the appearance of its heart on the fourth day. In fact some of his observations were only confirmed by zoologists in the 19th century and had for long been thought to be as erroneous as his physics.

In embryology he was also able to refute by dissection the prevailing view that the sex of an embryo is determined by its site in the womb. He also argued against the doctrine of pangenesis, that the seed comes from the whole of the body, as he also did against the classical version of preformationism, that the embryo contains all parts already preformed. His physiology, which could not be obtained so readily from simple dissection, was less acute. Respiration was thought to cool the body, an exercise unnecessary for fish who could cool themselves merely by drawing water through their gills.

He, further, produced a rudimentary taxonomy that went to some length to show that divisions based on number of limbs turned out to be obviously arbitrary. Instead, he proposed that mode of reproduction be used. This gave him the basic division between viviparous (exclusively mammalian) and the oviparous, subdivided into birds and reptiles laying proper eggs and the fishes laying ‘imperfect eggs’. He added the insects, who lay no eggs at all but simply produce larvae.

If Aristotle had produced only his Organon – works on logic – he would have been considered a prolific and powerful thinker. His style of logic lasted unchallenged even longer than his physics for it was not until 1847 that George Boole laid the foundations of a more modern logic and it was not until the present century that non-Aristotelian logics were systematically developed.