Before attempting to describe Greek philosophy, we must begin by answering a preliminary objection: is it even proper to speak of such a thing as “Greek philosophy,” or is this merely a conventional category created by academics to make their own work easier? These Greek philosophers lived hundreds of years and thousands of miles apart. They wrote on seemingly disparate topics ranging from cosmology to ethics. Eleatics and Ionians, Platonists and Pythagoreans, Stoics and Cynics faced off, haranguing one another; would we lump these groups under one designation? It would seem that the only thing tying them together is their common use of the Greek language.
Such a view would be mistaken. (Let us hope that it is a straw man and that no one actually holds to this position.) Though they lived in different times, their ideas endured. Though they lived in various places around the Mediterranean, travel was frequent. Though many focused on particular topics, all were concerned with answering the most fundamental questions of existence. Though they aligned themselves into opposing groups, they all engaged in the Great Conversation.
This term perhaps best describes what is at the heart of the philosophical enterprise that took place in the ancient Greek-speaking world. All of these men, in some form or facet, took up the question: “What is reality like, and how can I bring myself in line with it so as to have a happy life?” This question contains three key suppositions common to Greek thinkers of the period. All assumed that there was an order to the cosmos; reality was a coherent, unified whole. All assumed that this reality was intelligible, to some degree, by human reason (few outliers such as Gorgias notwithstanding). And all assumed that being in sync with reality was necessary for living a good life. While philosophers had different answers to this question, they were all fundamentally engaging it, and thus were engaged in the same conversation.
This belief in the power of reason to apprehend the nature of things is of particular importance. It creates a space separate from mythology in which to contend with the questions of existence. The philosopher is one who seeks an account of reality distinct from that which the storyteller or oracle can provide. The philosopher uses rational investigation to attempt to answer the great question.
(Note: "Mythology" and "philosophy" are not exhaustive categories; it's not the case that whatever is not philosophy is "mythology." Apart from mythology (storytelling) and philosophy (analytical reasoning), there are other categories, like "science," (empirical reasoning) or "divine revelation" (given knowledge), which, along with philosophy, are ways of gaining true knowledge about reality. But as these are not categories of thought for the Greeks--what we would call "science" they would call "natural philosophy"--I do not discuss them here.
Philosophers in different times and places were interested in different aspects of the question. For the Ionians and Eleatics, the first concern was the nature of reality as concerns its composition: what is everything made of? Behind this question was the assumption that, since we perceive the world to be a unified whole, it must thus be composed, at its base, of a single substance. Various substances were proposed: Thales said water; Anaximenes said air (in various states of condensation); Heraclitus said fire, in its constant flux; Anaximander suggested “the unlimited.” But all maintained that there must be substantial unity, even if this prime substance changes into different things.
The Eleatics heard this speculation and focused on the question of how such changes could occur. For Parmenides and his disciple Zeno, the answer was simple: they don’t. Though things appear to change, in principle they could not, for where would the new thing come from? How could what is come from what is not? They concluded that change was illusory. Though opposed to one another, the two schools at least agreed on one point: things were not precisely as they seemed.
It should be remembered that these thinkers, apart from their cosmological speculations, were concerned with ethical questions as well; it is not as though Thales was consigned to the natural philosophy department, away from the ethicists, forbidden to tackle their topics. But they did tend to be preoccupied with cosmological questions, just as many later thinkers, particularly in the Hellenistic and Roman periods, primarily addressed ethical questions. Some, like Pythagoras, Plato and Socrates, and Aristotle, did a little of everything, yet always in conversation with those who had gone before them.
Socrates occupies the place that he does in the history of philosophy because his thought has been the catalyst for so much of the conversation that has followed. Indeed, in Plato’s Socratic dialogues, we see precisely that: a conversation! The Socratic dialogue is a microcosm, a snapshot, of the whole of Greek philosophy: a conversation in which the thought of various people is engaged, questioned, expounded, examined, and cross-examined. This is best seen in those dialogues which feature other great philosophers, such as Gorgias, Parmenides, and Protagoras; here, we most literally see the Great Conversation happening before our eyes.
In these dialogues, Plato and Socrates wrestle with many of the most profound sub-questions which are part of that main question, “What is reality and how can I conform myself to it so as to have a happy life?” They addressed questions such as: what is knowledge? What is virtue? What is the relationship between the two? What is the nature of the cosmos? Of love? How is the polis best ordered so as to lead people to the good life? In Plato’s dialogues we see the interconnectedness of the varied facets of the conversation. Knowledge leads to virtue; knowledge requires education and formation of the soul; education requires a well-ordered society; yet a well-ordered society will not come about without virtuous inhabitants. Plato and Socrates show the unity of the philosophical enterprise, the unity of wisdom.
Aristotle took up this view and expanded it. Any subject, be it poetics, rhetoric, biology, physics, metaphysics, or ethics, was susceptible to philosophical inquiry, for all were part of the same cosmic order. Anything, from plays to porcupines, from substances to souls, from happiness to the heavens, could be analyzed according to four causes: what is it? What is it made of? What brought it to be? What is its end or purpose? And always, before presenting his conclusion, Aristotle would give due consideration to the theories of predecessors and contemporaries; he did not dismiss them with a wave of the hand, but took the time to attempt refuting them. He was engaged in the conversation.
Over time, the conversation shifted according to the predilections of those involved in it. Plato the geometer approached things one way, Aristotle the biologist another. Thales the engineer had one viewpoint, Pythagoras the near-mystic another. Likewise, circumstances in society had an effect. A citizen of an independent city-state will have different concerns from a subject of a king or emperor. After the Macedonian conquest of Greece, and later during the Roman period, a shift takes place: the philosopher becomes less concerned with the form of society than the ethical status of the individual. Yet even so, the conversation continued. The Epicureans and Stoics still looked back to Socrates as an inspiration of sorts, and engaged his ideas on the nature of the good.
One thought leads to another. One idea sparks a response, and that response prompts a counter-response. This is the nature of conversation, and it is the nature of Greek philosophy as it developed over hundreds of years, through all parts of the Greek-speaking world. That conversation continued on, through the Late Antique period, the Middle Ages, the Enlightenment, even today; as Alfred North Whitehead said, "All philosophy is but a footnote to Plato"--or rather, the whole Western philosophical tradition is the child of these Greeks.