Philosophy of nature

These philosophers defined all things by their quintessential substance (which Aristotle, perhaps being anachronistic, called the ἀρχή / arche) of which the world was formed and which was the source of everything. Thales thought it to be water. But as it was impossible to explain some things (such as fire) as being composed of this element, Anaximander chose an unobserved, undefined element, which he called apeiron (ἄπειρον "having no limit"). He reasoned that if each of the four traditional elements (Water, Air, Fire, and Earth) are opposed to the other three, and if they cancel each other out on contact, none of them could constitute a stable, truly elementary form of matter. Consequently, there must be another entity from which the others originate, and which must truly be the most basic element of all. The notion of temporal infinity was familiar to the Greek mind in the religious conception of immortality and Anaximander's description was in terms appropriate to this conception.This arche is called "eternal and ageless"(Hippolitus I,6,I;DK B2). The unspecified nature of the apeiron upset critics, which caused Anaximenes to define it as being air, a more concrete, yet still subtle, element.Anaximenes held that by its evaporation and condensation, air can change into other elements or substances such as fire, wind, clouds, water, and earth. However, our modern concept of energy is much more similar to Anaximander's apeiron.

Pythagoreanism was the system of esoteric and metaphysical beliefs held by Pythagoras and his followers, the Pythagoreans, who were considerably influenced by mathematics, music and astronomy. Pythagoreanism originated in the 5th century BC and greatly influenced platonism and the concept of vegetarianism. Later revivals of Pythagorean doctrines led to what is now called neopythagoreanism.

Heraclitus of Ephesus (/ˌhɛrəˈklaɪtəs/; Greek: Ἡράκλειτος ὁ Ἐφέσιος, Hērákleitos ho Ephésios; c. 535 – c. 475 BCE) was a pre-Socratic Greek philosopher, a native of the Greek city Ephesus, Ionia, on the coast of Asia Minor. He was of distinguished parentage. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom. From the lonely life he led, and still more from the riddling and paradoxical nature of his philosophy and his stress upon the needless unconsciousness of humankind, he was called "The Obscure" and the "Weeping Philosopher".

Heraclitus is famous for his insistence on ever-present change in the universe, as stated in the famous saying, "No man ever steps in the same river twice" (see panta rhei, below). He believed in the unity of opposites, stating that "the path up and down are one and the same", all existing entities being characterized by pairs of contrary properties. His cryptic utterance that "all entities come to be in accordance with this Logos" (literally, "word", "reason", or "account") has been the subject of numerous interpretations.

The Eleatics refers to the pre-Socratic school of philosophy founded by Parmenides in the early fifth century BC in the ancient town of Elea. Other members of the school included Zeno of Elea and Melissus of Samos. Xenophanes is sometimes included in the list, though there is some dispute over this. Elea, whose modern-day appellation is Velia, was a Greek colony located in present-day Campania in southern Italy. The school took its name from Elea (/ˈɛliə/; Greek: Ἐλέα), a Greek city of lower Italy, the home of its chief exponents, Parmenides and Zeno. Its foundation is often attributed to Xenophanes of Colophon, but, although there is much in his speculations which formed part of the later Eleatic doctrine, it is probably more correct to regard Parmenides as the founder of the school.

Xenophanes espoused a belief that "God is one, supreme among gods and men, and not like mortals in body or in mind." Parmenides developed some of Xenophanes's metaphysical ideas. Subsequently the school debated the possibility of motion and other such fundamental questions. The work of the school was influential upon Platonic metaphysics.

The Eleatics rejected the epistemological validity of sense experience, and instead took logical standards of clarity and necessity to be the criteria of truth. Of the members, Parmenides and Melissus built arguments starting from indubitably sound premises. Zeno, on the other hand, primarily employed the reductio ad absurdum, attempting to destroy the arguments of others by showing that their premises led to contradictions (Zeno's paradoxes).

The main doctrines of the Eleatics were evolved in opposition to the theories of the early physicalist philosophers, who explained all existence in terms of primary matter, and to the theory of Heraclitus, which declared that all existence may be summed up as perpetual change. The Eleatics maintained that the true explanation of things lies in the conception of a universal unity of being. According to their doctrine, the senses cannot cognize this unity, because their reports are inconsistent; it is by thought alone that we can pass beyond the false appearances of sense and arrive at the knowledge of being, at the fundamental truth that the "All is One". Furthermore, there can be no creation, for being cannot come from non-being, because a thing cannot arise from that which is different from it. They argued that errors on this point commonly arise from the ambiguous use of the verb to be, which may imply actual physical existence or be merely the linguistic copula which connects subject and predicate.

Though the conclusions of the Eleatics were rejected by the later Presocratics and Aristotle, their arguments were taken seriously, and they are generally credited with improving the standards of discourse and argument in their time. Their influence was likewise longlasting; Gorgias, a Sophist, argued in the style of the Eleatics in On Nature or What Is Not, and Plato acknowledged them in the Parmenides, the Sophist and the Statesman. Furthermore, much of the later philosophy of the ancient period borrowed from the methods and principles of the Eleatics.

Atomism (from Greek ἄτομον, atomon, i.e. "uncuttable", "indivisible") is a natural philosophy that developed in several ancient traditions. The atomists theorized that nature consists of two fundamental principles: atom and void. Unlike their modern scientific namesake in atomic theory, philosophical atoms come in an infinite variety of shapes and sizes, each indestructible, immutable and surrounded by a void where they collide with the others or hook together forming a cluster. Clusters of different shapes, arrangements, and positions give rise to the various macroscopic substances in the world.

· Milesian School

Thales (624 – c 546 BCE)

Anaximander (610 – 546 BCE)

Anaximenes of Miletus (c. 585 – c. 525 BCE)

· Pythagoreans

Pythagoras (582 – 496 BCE)

Philolaus (470 – 380 BCE)

Alcmaeon of Croton

Archytas (428 – 347 BCE)

· Heraclitus (535 – 475 BCE)

· Eleatic School

Xenophanes (570 – 470 BCE)

Parmenides (510 – 440 BCE)

Zeno of Elea (490 – 430 BCE)

Melissus of Samos (c. 470 BCE – ?)

· Pluralists

Empedocles (490 – 430 BCE)

Anaxagoras (500 – 428 BCE)

· Atomists

Leucippus (first half of 5th century BCE)

Democritus (460 – 370 BCE)

Metrodorus of Chios (4th century BCE)

· Pherecydes of Syros (6th century BCE)

· Sophists

Protagoras (490 – 420 BCE)

Gorgias (487 – 376 BCE)

Antiphon (480 – 411 BCE)

Prodicus (465/450 – after 399 BCE)

Hippias (middle of the 5th century BCE)

Thrasymachus (459 – 400 BCE)

Callicles

Critias

Lycophron

· Diogenes of Apollonia (c. 460 BCE – ?)