Greek Philosophers
The western philosophical tradition has been heavily shaped by a handful of Greek philosophical thinkers between the 6th century BC and the 6th century AD. Below are the most enduringly relevant and timeless philosophers from ancient and classical Greece.
Heraclitus
Heraclitus' main body of work is a cryptic one that forces his reader to engage and think a great deal. The crux of his work centers around the the notion of logos, or the idea that the universe was rationally organized and therefore understandable. This notion proposes a series of paradoxes, but Heraclitus, among other sages, contended that oppositions are in themselves forms of organization and can be part of a unified whole.
Democritus of Abdera
Democritus' main contention dealt with the notion that matter is made of atoms. At the time, there was no conclusive evidence of this, but it explained a good deal of matter's characteristics. In this theory, atoms are always moving and changing clusters in various, transient combinations, so things that existed may change, but nothing that is not already existing may come into being.
Socrates
The contemporaries of Socrates were often more interested in the diversity and nature of matter, but Socrates was more interested in ethics. He wrote nothing in his life because he believed that knoeldge was a living and interactive thing. He is perhaps most famous for his methods of inquiry, which still serve as models for teachers today. Socrates posed questions upon people and their assertions, challenging them through questions of contradiction, and never taking a position himself. It is through this kind of didactic conversation that he believed truth could be pursued. One of his main premises is this: "If one knows good, he will do good. It follows then, that anyone who does anything wrong doesn't really know what the good is."
Plato
The most famous of Socrates' pupils, Plato is responsible for preserving much of what we know of Socrates through his written dialogues. Central to Plato's thoughts was the idea of forms. He attempted to explain change in the physical world like his contemporaries, but ultimately synthesized their ideas into something different and unique. His magnum opus is The Republic, in which he tackles the question of justice in the state and the individual, emphasizing good education to the elite and ruling classes. One of his most famous teachings is the Allegory of the Cave, which discusses the relationship of forms to reality.
Antisthenes
After Socrates, Antisthenes was the most important Athenian philosopher. He attempted to find out what the words we use mean, like friendship, love, courage etc., but he settled on the idea that these definitions were not possible. He was also very much engaged in the opposition of good and bad, what inclined a person to do bad when he knew what was good. In this area he found much contention with Plato.
Aristotle
Most of Aristotles writings are lost, but his lecture notes remain. He was a student of Plato, but their teachings on science differ in many ways. He agrees with Plato that the highest human faculty was reason, and a supreme activity was contemplation, but Aristotle was equally focused on the world surrounding man, not just the one within him. This included physics and mechanics and biology, which began a train of organized scientific inquiry that characterizes scientific studies today.
Epicurus
Epicurus was interested primarily in pleasure. His premises mainly include this: We live happiest when we are free from the pains of life, and the way to get to this state is through a virtuous life. He hypothesized that we do not know the gods or their intentions, nor do we know of the afterlife; therefore, it is useless to speculate about them. All we know is the present and what feels good to each of us as a human.
Plotinus
Like Plato, Plotinus contended that our world was a shadow of a higher world, which was ultimately the shadow of one god. Essentially, he believed our world was made of levels of reality. God on the highest, then the intellect, the soul, and matter. Our perceptions of the shadow are the lowest level of reality, and Plotinus thought that man would increasingly try to escape from the level of matter and perception into unity with god.