* Thales of Miletus (c. 624–546 BCE): The ultimate reality is water, and all things arise from and return to water.
* Anaximander (c. 611–546 BCE): The ultimate reality is the boundless (apeiron), from which all things arise and to which they return.
* Anaximenes (c. 585–528 BCE): The ultimate reality is air, from which all things arise and to which they return.
* Pythagoras (c. 570–495 BCE): The ultimate reality is numbers, and all things are composed of numbers.
* Heraclitus (c. 535–475 BCE): The ultimate reality is fire, and all things are in constant flux.
* Parmenides (c. 515–450 BCE): The ultimate reality is being, and change is impossible.
* Zeno of Elea (c. 490–430 BCE): The ultimate reality is being, and motion is impossible.
* Empedocles (c. 490–430 BCE): The ultimate reality is the four elements (earth, air, fire, and water), which combine in different ways to produce all things.
* Anaxagoras (c. 500–428 BCE): The ultimate reality is an infinite number of tiny particles (seeds), which combine in different ways to produce all things.
* Democritus (c. 460–370 BCE): The ultimate reality is atoms, which are indivisible, indestructible, and eternal.
Epistemology
* Protagoras (c. 480–411 BCE): The only reality is what is perceived by the individual, and there is no objective truth.
* Gorgias (c. 483–375 BCE): There is no reality, and even if there were, it would be impossible to know it.
* Socrates (c. 470–399 BCE): Knowledge is the result of rational inquiry and reflection, and the unexamined life is not worth living.
* Plato (c. 427–348 BCE): The ultimate reality is the world of Forms, which are perfect and eternal, and the physical world is a mere shadow of the world of Forms.
* Aristotle (384–322 BCE): Knowledge is the result of experience and observation, and the physical world is the ultimate reality.
Ethics
* Socrates (c. 470–399 BCE): The ultimate goal of human life is to achieve eudaimonia (happiness), which is the result of living in accordance with virtue and reason.
* Plato (c. 427–348 BCE): The ultimate goal of human life is to achieve the Form of the Good, which is the source of all value and meaning.
* Aristotle (384–322 BCE): The ultimate goal of human life is to achieve eudaimonia (happiness), which is the result of living in accordance with nature and reason.