Greek Philosophers

Anaximenes (Greek: Άναξιμένης) of Miletus (585 BCE - 528 BCE): held that the air, with its variety of contents, its universal presence, its vague associations in popular fancy with the phenomena of life and growth, is the source of all that exists.

Heraclitus (Greek: Ἡράκλειτος) of Ephesus (ca. 535 - 475 BCE) disagreed with Thales, Anaximander, and Pythagoras about the nature of the ultimate substance and claimed instead that everything is derived from the Greek classical element fire, rather than from air, water, or earth.

Empedocles (ca. 490 BCE – ca. 430 BCE) was a citizen of Agrigentum, a Greek colony in Sicily. Empedocles' philosophy is best known for being the origin of the cosmogenic theory of the four classical elements.

Anaxagoras of Clazomenae (ca. 500-428 BCE) regarded material substance as an infinite multitude of imperishable primary elements, referring all generation and disappearance to mixture and separation respectively.

Archelaus was a Greek philosopher of the 5th century BCE, born probably in Athens. He was a pupil of Anaxagoras, and is said by Ion of Chios (Diogenes Laërtius, ii. 23) to have been the teacher of Socrates.

Hippo (ca. 425 BCE) was native of Magna Graecia (Italy). Very little is known about him. He held that water was the principle of all things.

Diogenes (ca. 425 BCE) was a native of Apollonia, either the one in Crete or in Thrace. Like Anaximenes, he believed air to be the one source of all being.

Socrates (Greek: Σωκράτης,) (469 BC–399 BC) was a Classical Greek philosopher. Credited as one of the founders of Western philosophy, he is an enigmatic figure known only through the classical accounts of his students. Plato's dialogues are the most comprehensive accounts of Socrates to survive from antiquity.

Theory of Knowledge

Plato's theory of Forms and his theory of knowledge are so interrelated that they must be discussed together. Influenced by Socrates, Plato was convinced that knowledge is attainable. He was also convinced of two essential characteristics of knowledge.

First, knowledge must be certain and infallible. Second, knowledge must have as its object that which is genuinely real as contrasted with that which is an appearance only. Because that which is fully real must, for Plato, be fixed, permanent, and unchanging, he identified the real with the ideal realm of being as opposed to the physical world of becoming. One consequence of this view was Plato's rejection of empiricism, the claim that knowledge is derived from sense experience. He thought that propositions derived from sense experience have, at most, a degree of probability. They are not certain. Furthermore, the objects of sense experience are changeable phenomena of the physical world. Hence, objects of sense experience are not proper objects of knowledge.

Plato's own theory of knowledge is found in the Republic, particularly in his discussion of the image of the divided line and the myth of the cave. In the former, Plato distinguishes between two levels of awareness: opinion and knowledge. Claims or assertions about the physical or visible world, including both common sense observations and the propositions of science, are opinions only. Some of these opinions are well founded; some are not; but none of them counts as genuine knowledge. The higher level of awareness is knowledge, because there reason, rather than sense experience, is involved. Reason, properly used, results in intellectual insights that are certain, and the objects of these rational insights are the abiding universals, the eternal Forms or substances that constitute the real world.