The convention of terming those philosophers who were active prior to Socrates the pre-Socratics gained currency with the 1903 publication of Hermann Diels' Fragmente der Vorsokratiker, although the term did not originate with him. The term is considered philosophically useful because what came to be known as the "Athenian school" (composed of Socrates, Plato, and Aristotle) signaled a profound shift in the subject matter and methods of philosophy; Friedrich Nietzsche's thesis that this shift began with Plato rather than with Socrates (hence his nomenclature of "pre-Platonic philosophy") has not prevented the predominance of the "pre-Socratic" distinction.
The pre-Socratics were primarily concerned with cosmology, ontology and mathematics. They were distinguished from "non-philosophers" insofar as they rejected mythological explanations in favor of reasoned discourse.
Thales of Miletus, regarded by Aristotle as the first philosopher, held that all things arise from water. It is not because he gave a cosmogony that John Burnet calls him the "first man of science," but because he gave a naturalistic explanation of the cosmos and supported it with reasons. According to tradition, Thales was able to predict an eclipse and taught the Egyptians how to measure the height of the pyramids.
Thales inspired the Milesian school of philosophy and was followed by Anaximander, who argued that the substratum or arche could not be water or any of the classical elements but was instead something "unlimited" or "indefinite" (in Greek, the apeiron). He began from the observation that the world seems to consist of opposites (e.g., hot and cold), yet a thing can become its opposite (e.g., a hot thing cold). Therefore, they cannot truly be opposites but rather must both be manifestations of some underlying unity that is neither. This underlying unity (substratum, arche) could not be any of the classical elements, since they were one extreme or another. For example, water is wet, the opposite of dry, while fire is dry, the opposite of wet. Anaximenes in turn held that the arche was air, although John Burnet argues that by this he meant that it was a transparent mist, the aether. Despite their varied answers, the Milesian school was searching for a natural substance that would remain unchanged despite appearing in different forms, and thus represents one of the first scientific attempts to answer the question that would lead to the development of modern atomic theory; "the Milesians," says Burnet, "asked for the φύσις of all things."
Xenophanes was born in Ionia, where the Milesian school was at its most powerful, and may have picked up some of the Milesians' cosmological theories as a result. What is known is that he argued that each of the phenomena had a natural rather than divine explanation in a manner reminiscent of Anaximander's theories and that there was only one god, the world as a whole, and that he ridiculed the anthropomorphism of the Greek religion by claiming that cattle would claim that the gods looked like cattle, horses like horses, and lions like lions, just as the Ethiopians claimed that the gods were snubnosed and black and the Thracians claimed they were pale and red-haired.
Burnet says that Xenophanes was not, however, a scientific man, with many of his "naturalistic" explanations having no further support than that they render the Homeric gods superfluous or foolish. He has been claimed as an influence on Eleatic philosophy, although that is disputed, and a precursor to Epicurus, a representative of a total break between science and religion.
Pythagoras lived at roughly the same time that Xenophanes did and, in contrast to the latter, the school that he founded sought to reconcile religious belief and reason. Little is known about his life with any reliability, however, and no writings of his survive, so it is possible that he was simply a mystic whose successors introduced rationalism into Pythagoreanism, that he was simply a rationalist whose successors are responsible for the mysticism in Pythagoreanism, or that he was actually the author of the doctrine; there is no way to know for certain.
Pythagoras is said to have been a disciple of Anaximandar and to have imbibed the cosmological concerns of the Ionians, including the idea that the cosmos is constructed of spheres, the importance of the infinite, and that air or aether is the arche of everything. Pythagoreanism also incorporated ascetic ideals, emphasizing purgation, metempsychosis, and consequently a respect for all animal life; much was made of the correspondence between mathematics and the cosmos in a musical harmony.
Heraclitus must have lived after Xenophanes and Pythagoras, as he condemns them along with Homer as proving that much learning cannot teach a man to think; since Parmenides refers to him in the past tense, this would place him in the 5th century BCE. Contrary to the Milesian school, who would have one stable element at the root of all, Heraclitus taught that "everything flows" or "everything is in flux," the closest element to this flux being fire; he also extended the teaching that seeming opposites in fact are manifestations of a common substrate to good and evil itself.
Parmenides of Elea cast his philosophy against those who held "it is and is not the same, and all things travel in opposite directions,"—presumably referring to Heraclitus and those who followed him. Whereas the doctrines of the Milesian school, in suggesting that the substratum could appear in a variety of different guises, implied that everything that exists is corpuscular, Parmenides argued that the first principle of being was One, indivisible, and unchanging. Being, he argued, by definition implies eternality, while only that which is can be thought; a thing which is, moreover, cannot be more or less, and so the rarefaction and condensation of the Milesians is impossible regarding Being; lastly, as movement requires that something exist apart from the thing moving (viz. the space into which it moves), the One or Being cannot move, since this would require that "space" both exist and not exist. While this doctrine is at odds with ordinary sensory experience, where things do indeed change and move, the Eleatic school followed Parmenides in denying that sense phenomena revealed the world as it actually was; instead, the only thing with Being was thought, or the question of whether something exists or not is one of whether it can be thought.
In support of this, Parmenides' pupil Zeno of Elea attempted to prove that the concept of motion was absurd and as such motion did not exist. He also attacked the subsequent development of pluralism, arguing that it was incompatible with Being. His arguments are known as Zeno's paradoxes.
The power of Parmenides' logic was such that some subsequent philosophers abandoned the monism of the Milesians, Xenophanes, Heraclitus, and Parmenides, where one thing was the arche, and adopted pluralism, such as Empedocles and Anaxagoras. There were, they said, multiple elements which were not reducible to one another and these were set in motion by love and strife (as in Empedocles) or by Mind (as in Anaxagoras). Agreeing with Parmenides that there is no coming into being or passing away, genesis or decay, they said that things appear to come into being and pass away because the elements out of which they are composed assemble or disassemble while themselves being unchanging.
Leucippus also proposed an ontological pluralism with a cosmogony based on two main elements: the vacuum and atoms. These, by means of their inherent movement, are crossing the void and creating the real material bodies. His theories were not well known by the time of Plato, however, and they were ultimately incorporated into the work of his student, Democritus.
Sophistry arose from the juxtaposition of physis (nature) and nomos (law). John Burnet posits its origin in the scientific progress of the previous centuries which suggested that Being was radically different from what was experienced by the senses and, if comprehensible at all, was not comprehensible in terms of order; the world in which men lived, on the other hand, was one of law and order, albeit of humankind's own making. At the same time, nature was constant, while what was by law differed from one place to another and could be changed.
The first man to call himself a sophist, according to Plato, was Protagoras, whom he presents as teaching that all virtue is conventional. It was Protagoras who claimed that "man is the measure of all things, of the things that are, that they are, and of the things that are not, that they are not," which Plato interprets as a radical perspectivism, where some things seem to be one way for one person (and so actually are that way) and another way for another person (and so actually are that way as well); the conclusion being that one cannot look to nature for guidance regarding how to live one's life.
Protagoras and subsequent sophists tended to teach rhetoric as their primary vocation. Prodicus, Gorgias, Hippias, and Thrasymachus appear in various dialogues, sometimes explicitly teaching that while nature provides no ethical guidance, the guidance that the laws provide is worthless, or that nature favors those who act against the laws.
Socrates, born in Athens in the 5th century BCE, marks a watershed in ancient Greek philosophy. Athens was a center of learning, with sophists and philosophers traveling from across Greece to teach rhetoric, astronomy, cosmology, geometry, and the like. The great statesman Pericles was closely associated with this new learning and a friend of Anaxagoras, however, and his political opponents struck at him by taking advantage of a conservative reaction against the philosophers; it became a crime to investigate the things above the heavens or below the earth, subjects considered impious. Anaxagoras is said to have been charged and to have fled into exile when Socrates was about twenty years of age. There is a story that Protagoras, too, was forced to flee and that the Athenians burned his books. Socrates, however, is the only subject recorded as charged under this law, convicted, and sentenced to death in 399 BCE (see Trial of Socrates). In the version of his defense speech presented by Plato, he claims that it is the envy he arouses on account of his being a philosopher that will convict him.
While philosophy was an established pursuit prior to Socrates, Cicero credits him as "the first who brought philosophy down from the heavens, placed it in cities, introduced it into families, and obliged it to examine into life and morals, and good and evil." By this account he would be considered the founder of political philosophy. The reasons for this turn toward political and ethical subjects remain the object of much study.
The fact that many conversations involving Socrates (as recounted by Plato and Xenophon) end without having reached a firm conclusion, or aporetically, has stimulated debate over the meaning of the Socratic method. Socrates is said to have pursued this probing question-and-answer style of examination on a number of topics, usually attempting to arrive at a defensible and attractive definition of a virtue.
While Socrates' recorded conversations rarely provide a definite answer to the question under examination, several maxims or paradoxes for which he has become known recur. Socrates taught that no one desires what is bad, and so if anyone does something that truly is bad, it must be unwillingly or out of ignorance; consequently, all virtue is knowledge. He frequently remarks on his own ignorance (claiming that he does not know what courage is, for example). Plato presents him as distinguishing himself from the common run of mankind by the fact that, while they know nothing noble and good, they do not know that they do not know, whereas Socrates knows and acknowledges that he knows nothing noble and good.
Numerous subsequent philosophical movements were inspired by Socrates or his younger associates. Plato casts Socrates as the main interlocutor in his dialogues, deriving from them the basis of Platonism (and by extension, Neoplatonism). Plato's student Aristotle in turn criticized and built upon the doctrines he ascribed to Socrates and Plato, forming the foundation of Aristotelianism. Antisthenes founded the school that would come to be known as Cynicism and accused Plato of distorting Socrates' teachings. Zeno of Citium in turn adapted the ethics of Cynicism to articulate Stoicism. Epicurus studied with Platonic and Stoic teachers before renouncing all previous philosophers (including Democritus, on whose atomism the Epicurean philosophy relies). The philosophic movements that were to dominate the intellectual life of the Roman empire were thus born in this febrile period following Socrates' activity, and either directly or indirectly influenced by him. They were also absorbed by the expanding Muslim world in the 7th through 10th centuries AD, from which they returned to the West as foundations of Medieval philosophy and the Renaissance, as discussed below.
Plato was an Athenian of the generation after Socrates. Ancient tradition ascribes thirty-six dialogues and thirteen letters to him, although of these only twenty-four of the dialogues are now universally recognized as authentic; most modern scholars believe that at least twenty-eight dialogues and two of the letters were in fact written by Plato, although all of the thirty-six dialogues have some defenders. A further nine dialogues are ascribed to Plato but were considered spurious even in antiquity.
Plato's dialogues feature Socrates, although not always as the leader of the conversation. (One dialogue, the Laws, instead contains an "Athenian Stranger.") Along with Xenophon, Plato is the primary source of information about Socrates' life and beliefs and it is not always easy to distinguish between the two. While the Socrates presented in the dialogues is often taken to be Plato's mouthpiece, Socrates' reputation for irony, his caginess regarding his own opinions in the dialogues, and his occasional absence from or minor role in the conversation serve to conceal Plato's doctrines. Much of what is said about his doctrines is derived from what Aristotle reports about them.
The political doctrine ascribed to Plato is derived from the Republic, the Laws, and the Statesman. The first of these contains the suggestion that there will not be justice in cities unless they are ruled by philosopher kings; those responsible for enforcing the laws are compelled to hold their women, children, and property in common; and the individual is taught to pursue the common good through noble lies; the Republic says that such a city is likely impossible, however, generally assuming that philosophers would refuse to rule and the people would refuse to compel them to do so.
Whereas the Republic is premised on a distinction between the sort of knowledge possessed by the philosopher and that possessed by the king or political man, Socrates explores only the character of the philosopher; in the Statesman, on the other hand, a participant referred to as the Eleatic Stranger discusses the sort of knowledge possessed by the political man, while Socrates listens quietly. Although rule by a wise man would be preferable to rule by law, the wise cannot help but be judged by the unwise, and so in practice, rule by law is deemed necessary.
Both the Republic and the Statesman reveal the limitations of politics, raising the question of what political order would be best given those constraints; that question is addressed in the Laws, a dialogue that does not take place in Athens and from which Socrates is absent. The character of the society described there is eminently conservative, a corrected or liberalized timocracy on the Spartan or Cretan model or that of pre-democratic Athens.
Plato's dialogues also have metaphysical themes, the most famous of which is his theory of forms. It holds that non-material abstract (but substantial) forms (or ideas), and not the material world of change known to us through our physical senses, possess the highest and most fundamental kind of reality.
Plato often uses long-form analogies (usually allegories) to explain his ideas; the most famous is perhaps the Allegory of the Cave. It likens most humans to people tied up in a cave, who look only at shadows on the walls and have no other conception of reality. If they turned around, they would see what is casting the shadows (and thereby gain a further dimension to their reality). If some left the cave, they would see the outside world illuminated by the sun (representing the ultimate form of goodness and truth). If these travelers then re-entered the cave, the people inside (who are still only familiar with the shadows) would not be equipped to believe reports of this 'outside world'. This story explains the theory of forms with their different levels of reality, and advances the view that philosopher-kings are wisest while most humans are ignorant. One student of Plato (who would become another of the most influential philosophers of all time) stressed the implication that understanding relies upon first-hand observation:
Aristotle moved to Athens from his native Stageira in 367 BCE and began to study philosophy (perhaps even rhetoric, under Isocrates), eventually enrolling at Plato's Academy. He left Athens approximately twenty years later to study botany and zoology, became a tutor of Alexander the Great, and ultimately returned to Athens a decade later to establish his own school: the Lyceum. At least twenty-nine of his treatises have survived, known as the corpus Aristotelicum, and address a variety of subjects including logic, physics, optics, metaphysics, ethics, rhetoric, politics, poetry, botany, and zoology.
Aristotle is often portrayed as disagreeing with his teacher Plato (e.g., in Raphael's School of Athens). He criticizes the regimes described in Plato's Republic and Laws, and refers to the theory of forms as "empty words and poetic metaphors." He is generally presented as giving greater weight to empirical observation and practical concerns.
Aristotle's fame was not great during the Hellenistic period, when Stoic logic was in vogue, but later peripatetic commentators popularized his work, which eventually contributed heavily to Islamic, Jewish, and medieval Christian philosophy. His influence was such that Avicenna referred to him simply as "the Master"; Maimonides, Alfarabi, Averroes, and Aquinas as "the Philosopher."
During the Hellenistic and Roman periods, many different schools of thought developed in the Hellenistic world and then the Greco-Roman world. There were Greeks, Romans, Egyptians, Syrians and Arabs who contributed to the development of Hellenistic philosophy. Elements of Persian philosophy and Indian philosophy also had an influence. The most notable schools of Hellenistic philosophy were:Neoplatonism: Plotinus (Egyptian), Ammonius Saccas, Porphyry (Syrian), Zethos (Arab), Iamblichus (Syrian), Proclus
Academic Skepticism: Arcesilaus, Carneades, Cicero (Roman)
Pyrrhonian Skepticism: Pyrrho, Sextus Empiricus
Cynicism: Antisthenes, Diogenes of Sinope, Crates of Thebes (taught Zeno of Citium, founder of Stoicism)
Stoicism: Zeno of Citium, Cleanthes, Chrysippus, Crates of Mallus (brought Stoicism to Rome c. 170 BCE), Panaetius, Posidonius, Seneca (Roman), Epictetus (Greek/Roman), Marcus Aurelius (Roman)
Epicureanism: Epicurus (Greek) and Lucretius (Roman)
Eclecticism: Cicero (Roman)
The spread of Christianity throughout the Roman world, followed by the spread of Islam, ushered in the end of Hellenistic philosophy and the beginnings of Medieval philosophy, which was dominated by the three Abrahamic traditions: Jewish philosophy, Christian philosophy, and early Islamic philosophy.
During the Middle Ages, Greek ideas were largely forgotten in Western Europe (where, between the fall of Rome and the East-West Schism, literacy in Greek had declined sharply). Not long after the first major expansion of Islam, however, the Abbasid caliphs authorized the gathering of Greek manuscripts and hired translators to increase their prestige. Islamic philosophers such as Al-Kindi (Alkindus), Al-Farabi (Alpharabius), Ibn Sina (Avicenna) and Ibn Rushd (Averroes) reinterpreted these works, and during the High Middle Ages Greek philosophy re-entered the West through translations from Arabic to Latin. The re-introduction of these philosophies, accompanied by the new Arabic commentaries, had a great influence on Medieval philosophers such as Thomas Aquinas.