Aristotle of Stagira (384-322 BC)
Greek philosopher usually upheld as one of the greatest philosophers of all times. Aristotle studied at the Academy, but disagreed with Plato, feeling that one could obtain knowledge about the natural world. He distinguished between two types of philosophers: the physiologoi (natural philosophers) who study nature (e.g. Thales, Anaximander, and Anaximenes) and the theologoi who used gods and myths (e.g. Homer and Hesiod). Aristotle believed that there exists a "golden mean," or desirable middle ground between any two extremes. He founded his own school in Athens called the Lyceum (or "peripatetic school," since Aristotle used to lecture while walking) which emphasized natural philosophy. Aristotle's lectures were compiled into 150 volumes including Physics, Metaphysics, and De Caelo et Mundo (On the Heavens and Earth).
Aristotle philosophized on virtually every other subject. He classified animals in a "Scala Naturae" or "Chain of Being" which consisted of God, man, mammals, oviparous with perfect eggs (e.g., birds), oviparous with non-perfect eggs (e.g., fish), insects, plants, and non-living matter. He considered each link in the chain as a "species." He also made extensive taxonomic studies of more than 500 animal species, dissecting many of them. The observations he published in Generation of Animals and Historia Animalum (Investigation of Animals) were meticulous, and his classification scheme conspicuously modern, departing from the prior Greek practices of using categories such as with feet/footless and winged/wingless. Aristotle achieved such a feat in biology by making use of the same principles of logic (whose systematic study he was the founder of) that he applied in his physical investigations. He did not, however, make a real classification system for plants.
In De Caelo, a work on the heavens, Aristotle accepted the heavenly spheres of Eudoxus, thought the Earth to be spherical, and imagined a perfectly spherical unchanging universe centered on theEarth. In trying to make a mechanical model of the crystalline spheres, he was forced to introduce "reacting spheres." His model, however, still did not account for variations in the planets' brightnesses. Aristotle suggested a tetrad of elements: earth (solid), fire (energy), water (liquid), and air (gas). Aristotle believed each element could be hot, wet, dry, or cold. He believed earth and heaven to be subject to two different sets of laws. Terrestrial dynamics consisted of natural (straight line, vertical, discontinuous, towards natural place, speed proportional to weight over density) and forced (discontinuous, towards natural place, speed proportional to force over weight) motion. Celestial dynamics was always natural, circular, and continuous. He also rejected the atomic theories ofDemocritus.
In his works, he examined the opinions of earlier thinkers. He believed that all events have material (object which is changed), formal (properties of the object which is changed), efficient (agent responsible for change), and final (reason for change) causes. In the twelfth and thirteenth centuries A. D., Aristotle's works came to be accepted as absolute truths, a fact which served to effectively stifle original and experiment-based scientific progress for centuries.