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Philosophy 2.0

Comprehensive LibGuide for philosophy, covering world philosophy, branches of philosophy, movements, and concepts.

[Ancient Greek Philosophy] flourished for a period of more than 1,000 years, from before 600 BCE to soon after CE 600, by when all or nearly all the philosophical approaches of the Western world, and many of the scientific theories that have become accepted since then – among them, the atomic theory, the heliocentric view of the solar system, and the theory of organic evolution – had been outlined.

These developments took place in Schools of philosophy that developed in Ancient Greece or Hellas, a culturally cohesive but politically fragmented area in what is now Greece and some of the surrounding countries. Each governmental unit in this area was a polis or city-state, whose internal organization varied widely from polis to polis, and from time to time. Athens, for example, was a direct democracy at one time, but not at most other times. Sparta always had a governmental organization widely different from that of Athens, with which it was frequently at war.

1. Greek philosophy. (2001). In A. P. Iannone, Dictionary of world philosophy. Routledge.

Introductory and In-Depth Material

History of Ancient Greek Philosophy

Ancient Greek Philosophy

Classical philosophy emerged in ancient Greece, following a procession from what are known as the Presocratics; to the three great philosophers, Socrates (470–399 BCE), Plato (c. 428–347 BCE), and Aristotle (384–322 BCE); and then to later schools of thought, including the Epicureans and Stoics. As is the case with all ancient societies, knowledge of these thinkers is limited by the documentation that has survived. Socrates, for example, wrote down nothing. Rather, Plato wrote dialogues featuring his mentor Socrates engaged in philosophical debate with various individuals in Athens, some of them his fellow citizens and other prominent visitors to the city. The material that has survived from ancient Greece has fueled philosophical discourse for two millennia.

The Presocratics

The term Presocratics is somewhat problematic. At least a few of the thinkers considered part of this school were contemporaries of Socrates and are mentioned in Plato’s dialogues. Foremost among these are the Sophists, traveling teachers of rhetoric who serve as foils for Plato’s philosophers. Plato sought to distinguish philosophers, seekers of truth, from Sophists, whom he regarded as seeking wealth and fame and peddling in fallacious arguments. Indeed, one of the most prominent Sophists, Protagoras, is a main character in the dialogue that bears his name.

Researching the Presocratics is difficult because so little of their work has survived. What we have is fragmentary and often based on the testimony of later philosophers. Still, based on the work that is available, we can characterize the Presocratics as interested in questions of metaphysics and natural philosophy, with many of them proposing that nature consisted of one or more basic substances.

The fragments of the works of these early philosophers that have come down to us focus on metaphysical questions. One of the central debates among the Presocratics is between monism and plurism. Those who think nature consisted of a single substance are called monists, in contrast to pluralists, who see it as consisting of multiple substances. For example, the monist Thales of Miletus thought that the basic element that comprised everything was water, while Empedocles the pluralist sought to show that there were four basic elements (earth, air, fire, and water) that were resolved and dissolved by the competing forces of love and strife.

Two panels containing symbols. The left panel, labelled Monism, contains a sketch of water. The right panel, labelled Pluralism, is split into four sections, each containing a different image: earth, air, water, fire.

Figure 4.3 A central debate among PreSocratic Greek philosophers concerned whether nature consisted of a single substance—an approach taken by the monists—or was made up of a number of substances—a position taken by the pluralists. One prominent monist, Thales of Miletus, posited that all of nature was made of water. Empedocles, a pluralist, argued instead that the four elements of earth, air, fire, and water formed the basis of the natural world. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Prominent Monists

Presocratic philosophers who sought to present a unified conception of nature held that nature ultimately consists of a single substance. This proposition can be interpreted in various ways. The claim proposed by Thales of Miletus (620–546 BCE) that the basic substance of the universe was water is somewhat ambiguous. It might mean that everything is ultimately made of water, or it might mean that water is the origin of all things. Thales and two of his students, Anaximander and Anaximenes, made up the monist Milesian school. Anaximander thought that water was too specific to be the basis for everything that exists. Instead, he thought the basic stuff of the universe was the apeiron, the indefinite or boundless. Anaximenes held that air was the basic substance of the universe.

Parmenides, one of the most influential Presocratic monists, went so far as to deny the reality of change. He presented his metaphysical ideas in a poem that portrays himself being taken on a chariot to visit a goddess who claims she will reveal the truths of the universe to him. The poem has two parts, “the Way of the Truth”, which explains that what exists is unified, complete, and unchanging, and “the Way of Opinion”, which argues that the perception of change in the physical world is mistaken. Our senses mislead us. Although it might seem to us that Parmenides’s claim that change is not real is absurd, he and his student Zeno advanced strong arguments. Parmenides was the first person to propose that the light from the moon came from the sun and to explain the moon’s phases. In this way, he showed that although we see the moon as a crescent, a semicircle, or a complete circle, the moon itself does not change (Graham 2013). The perception that the moon is changing is an illusion.

Zeno proposed paradoxes, known as Zeno’s paradoxes, that demonstrate that what we think of as plurality and motion are simply not possible. Say, for example, that you wish to walk from the library to the park. To get there, you first must walk halfway there. To finish your trip, you must walk half of the remaining distance (one quarter). To travel that final quarter of the distance, you must first walk half of that (an eighth of the total distance). This process can continue forever—creating an infinite number of discrete distances that you must travel. It is therefore impossible that you arrive at the park. A more common way to present this paradox today is as a mathematical asymptote or limit (Figure 4.4). From this point of view, you can never reach point a from point b because no matter where you are along the path, there will always be a distance between wherever you are and where you want to be.

Graph with x- and y-axes and curved lines in the 2nd and 3rd quadrants, which nearly touch the axes at each end and curve away in the middle. Red dotted lines run parallel to the axes. The line running along the y-axis is labelled “Vertical asymptote.” The line running parallel to the x-axis is labelled “Horizontal asymptote.” Although the curved lines nearly touch the asymptotes, they never fully reach them.

Figure 4.4 For the function y = 1/x, neither x nor y can have a value of zero because y approaches infinity as x approaches zero and x approaches infinity as y approaches zero. Other functions show these same characteristics, which are called asymptotes or limits. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Prominent Pluralists

Parmenides and Heraclitus (525–475 BCE) held diametrically opposed views concerning the nature of the universe. Where Parmenides saw unity, Heraclitus saw diversity. Heraclitus held that nothing remains the same and that all is in flux. One of his most well-known sayings illustrates this well: “[It is not possible to step twice into the same river]. . . . It scatters and again comes together, and approaches and recedes” (quoted in Curd 2011, 45).

Anaxagoras (500–428 BCE) and Empedocles (494–434 BCE) were substance pluralists who believed that the universe consisted of more than one basic kind of “stuff.” Anaxagoras believed that it is mind, or nous, that controls the universe by mixing and unmixing things into a variety of different combinations. Empedocles held that there were four basic substances (the four elements of air, earth, fire, and water) that were combined and recombined by the opposing forces of love and strife.

Finally, there are the schools of the atomists, who held the view that the basic substance of the universe was tiny, indivisible atoms. For the atomists, all was either atoms or void. Everything we experience is a result of atoms combining with one another.

Presocratic Theology

The Presocratic philosopher Pythagoras (570–490 BCE) and his followers, known as the Pythagoreans, comprised a rational yet mystical sect of learned men. The Pythagoreans had a reputation for learning and were legendary for their knowledge of mathematics, music, and astronomy as well as for their dietary practices and other customs (Curd 2011). Like Socrates, Pythagoras wrote nothing, so scholars continue to debate which ideas originated with Pythagoras and which were devised by his disciples.

Among the Pythagoreans’ key beliefs was the idea that the solution to the mysteries of the universe was numerical and that these numerical mysteries could be revealed through music. A reminder of their mathematical legacy can be found in the Pythagorean theorem, which students continue to learn in school. Pythagoreans also believed in the transmigration of souls, an idea that Plato would adopt. According to this doctrine, the soul outlives the body, and individuals are reborn after death in another human body or even in the body of a nonhuman animal.

Another important Presocratic philosopher who produced novel theological ideas is Xenophanes (c. 570–478 BCE). Xenophanes, who was fascinated by religion, rejected the traditional accounts of the Olympian gods. He sought a rational basis of religion and was among the first to claim that the gods are actually projections of the human mind. He argued that the Greeks anthropomorphized divinity, and like many later theologians, he held that there is a God whose nature we cannot grasp.

Socrates and Plato

As Socrates never wrote anything, he is remembered today because thinkers like Plato featured him in their writings. Plato deliberately dramatized the life of his teacher Socrates. One of the key questions of Plato’s scholarship is exactly how many liberties he took in depicting the life of his teacher. Scholars generally agree that the dialogues that Plato wrote early in his career are more faithful to the life of Socrates than later ones. His writings are usually divided into three periods: early, middle, and late.

The early dialogues feature a skeptical Socrates who refuses to advance any doctrines of his own. Instead, he questions his interlocutors until they despair of finding the truth at all. These early dialogues tend to be somewhat short with a simpler composition. One of the dialogues features a young man named Meno who is the pupil of a prominent Sophist. The dialogue focuses on the nature of virtue and whether virtue can be taught. At one point in the dialogue, Meno famously compares Socrates to a torpedofish, a fish similar to a stingray that paralyzes its prey. Socrates does this to his dialogue partners: they begin the discussion believing that they know something and over the course of the dialogue begin to question whether they know anything at all.

Gradually, Plato has Socrates give voice to more positive doctrines. These include what comes to be known as the theory of the forms, a metaphysical doctrine that holds that every particular thing that exists participates in an immaterial form or essence that gives this thing its identity. The invisible realm of the forms differs fundamentally from the changing realm we experience in this world. The invisible realm is eternal, unchanging, and perfect. The material things themselves change, but the immaterial forms remain the same. Consider, for example, the form of a rectangle: four adjacent straight sides that meet at 90-degree angles. You can draw a rectangle, but it is an imperfect representation. The desk or table you are sitting at might be rectangular, but are its edges perfectly straight? How perfect was the instrument that cut the sides? If you nick the edge of a table, then it changes and becomes less like the form of a rectangle. With the doctrine of forms, Plato may be said to combine the metaphysics of Parmenides with that of Heraclitus into a metaphysical dualism.

The philosopher’s task is to access the immaterial realm of the forms and try to convince others of its truth. Plato further believed that if we understand the true nature of virtues like wisdom, justice, and courage, we cannot avoid acting in accordance with them. Hence, rulers of states should be philosopher-kings who have the clearest understanding of forms. Yet philosopher-kings never have perfect knowledge because our understanding is based on a material realm that is always changing. True knowledge is only possible in the abstract realms, such as math and ethics.

In the dialogues, Socrates claims that he was divinely inspired to question prominent citizens of Athens to determine whether their claims to know could be verified. These citizens grow annoyed with Socrates after some years of this treatment, eventually bringing charges against him for corrupting the youth and making the weaker argument appear the stronger. The proceedings of the resulting trial were immortalized in Plato’s Apologia, where Socrates presents his defense of his life’s work as a philosopher. The dialogue’s name derives from the Greek apologia, meaning “defense”—Socrates never apologizes for anything! He is found guilty and sentenced to death. Socrates becomes a martyr to philosophy, put to death by the democratic government of Athens.


During the Middle Ages, people referred to Plato’s most famous pupil Aristotle as simply “the Philosopher.” This nickname is a testament to his enduring fame, as well as to the fact that he was driven by philosophical curiosity to try to understand everything under the sun. The first sentence of his famous work Metaphysics states, “Philosophy begins in wonder.” He exemplified this claim in his writing. His works ranged widely across all the main areas of philosophy, including logic, metaphysics, and ethics. In addition, he investigated natural philosophy, the fields of study that eventually gave rise to science. Aristotle also researched topics that would today be classified as biology and physics. Stylistically, his work was very different from that of his teacher. While Plato’s work was literary and even dramatic, Aristotle’s writings are presented as lecture.

Plato and his successors were prone to mysticism. It was easy to translate the philosophical theory of the forms into a mystical doctrine in which the forms were known by the mind of God. Aristotle resisted this trend. At the center of Aristotle’s work was his doctrine of the four causes. He believed that the nature of any single thing could be understood by answering four basic questions: “What’s it made of?” (material cause), “What shape does it have?” (formal cause), “What agent gave it this form?” (efficient cause), and, finally, “What is its end goal?” (final cause). Not only can we explain the nature of anything by answering these four basic questions, we can also understand the nature of the universe. Aristotle’s universe is a closed system that is comprehensible to humanity because it is composed of these four causes. Each cause leads to another, until we get to the first cause or prime mover at the head of it all. Somewhat obscurely, Aristotle claims that this first cause is “thought thinking itself.”

In addition to the doctrine of the four causes, it is important to understand Aristotle’s account of the soul. Unlike Plato, who held that the soul is an eternal substance that is reborn in various bodies, Aristotle has a functional conception of the soul. He defined the soul based upon what the soul does. In Aristotle’s understanding, all living things have souls. Plants have a vegetative soul that promotes growth and the exchange of nutrients. The animal soul, in addition to taking in nutrients and growing, experiences the world, desires things, and can move of its own volition. Added to these various functions in humans is the ability to reason.

Three panels, the first containing a sketch of a plant, the second a picture of a deer, and the third an outline of a human being.

Figure 4.5 Aristotle believed that all living beings had souls, but that the souls of various types of creatures differed in their abilities. The soul of a plant promotes growth and the exchange of nutrients. The animal soul allows for everything a plant can do, with the additional ability to desire things and move of its own volition. Only the human soul makes possible the ability to reason. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

With the four causes and the functional conception of the soul, we can begin to understand Aristotle’s ethics. Aristotle systematized Plato’s conception of ethics based upon his conception of the self and his four causes. Since everything that exists has a purpose, one of the basic questions for ethics is “What is the purpose of the human being?” After considering such candidates as pleasure and power, Aristotle settles on the answer “happiness” or, more accurately, “eudaimonia.” Rather than a fleeting emotional state, eudaimonia is better understood as “flourishing.” So the question at the heart of Aristotle’s ethics is “How should humans best achieve happiness?” His basic answer is that we achieve eudaimonia by cultivating the virtues. Virtues are habits of character that help us to decide what action is preferable in a particular moment. Cultivating these virtues will helps us to lead a fulfilling life.

It is generally true to say that Plato tended to be more focused on the transcendental world of the forms while Aristotle and his followers were more focused on this worldly existence. They shared a belief that the universe was comprehensible and that reason should serve as a guide to ordering our lives.


In the wake of the giants of Greek philosophy—Socrates, Plato, and Aristotle—some philosophers turned away from Plato’s ideal forms and toward materialism. In this, they can be seen as furthering a trend already present in the thinking of Aristotle. For Aristotle, there can be no immaterial forms—everything that exists has some material basis, though he allows an exception for his first cause, the unmoved mover.

The Epicureans steadfastly rejected the existence of immaterial forms, unmoved movers, and immaterial souls. The Epicureans, like Aristotle, embraced empiricism, which means that they believed that all knowledge was derived from sense experience. This view was the basis of the revival of empiricism in 18th-century British thought and scientific practice. They espoused an ethical naturalism that held that in order to live a good life we must properly understand human nature. The ultimate goal of life is to pursue pleasure. Despite their disagreements with Plato and, to a lesser extent, Aristotle, the Epicureans agreed with their predecessors that human existence ought to be guided by reason.

The two principal Greek Epicureans were Epicurus himself (341–270 BCE) and his Roman disciple Lucretius (c. 99–55 BCE). Although Epicurus’s views are characterized as hedonistic, this does not mean that he believed that we ought to be indiscriminate pleasure-seekers. Instead, he proposed that people could achieve fulfilling lives if they were self-sufficient and lived free from pain and fear. Of course, complete self-sufficiency is just as impossible as a life utterly free from pain and fear, but Epicurus believed that we should strive to minimize our dependence upon others while limiting the pain in our lives. Epicureans thought that the best way to do this was to retire from society into philosophical communities far from the hustle and bustle of the crowd. Epicurus and Lucretius saw the fear of death as our most debilitating fear, and they argued that we must overcome this fear if we were going to live happy lives.

Lucretius developed Epicurean philosophy in a poem called De Rerum Natura (On the nature of things). This poem discusses ethical ideas, but physics provides its focus. Lucretius adopts a material atomism that holds that things are composed of atoms in motion. Rejecting religious explanations, he argues that the universe is governed by chance and exemplified by these atoms in motion. Although the Epicurean philosophers were critically responding to the work of Plato and Aristotle, it should be evident that they also have antecedents in Presocratic thought. We can see this in their atomism and their religious skepticism, which hearkens back to Xenophanes.

“Introduction to Philosophy” by Smith, Browne, Conkling, Friedman, Fritz, Garro, Gallegos, Gill, Horton, Bosco, Longtin, McCall, Stuke, used under CC BY 4.0/ Elements edited

Socrates, Plato, and Aristotle

1. Weber, E. (2015). Socrates, Plato, and Aristotle (short documentary). YouTube. Retrieved from