Greek philosophy seems to begin with a preposterous fancy, with the proposition that water is the origin and mother-womb of all things. Is it really necessary to stop there and become serious? Yes, and for three reasons: firstly, because the preposition does enunciate something about the origin of things; secondly, because it does so without figure and fable; thirdly and lastly, because it contained, although only in the chrysalis state, the idea :everything is one. ..That which drove him (Thales) to this generalization was a metaphysical dogma, which had its origin in a mystic intuition and which together with the ever renewed endeavors to express it better, we find in all philosophies- the proposition: everything is one! (Nietzsche, 1890)

Greek philosophers
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	We know almost nothing about Thales of Miletus. Later generations told many anecdotes about this wise man, but it is difficult to verify the reliability of these stories. What seems certain, however, is that he predicted the solar eclipse of 28 May 585, which was remembered because the Lydian king Alyattes and the Median king Cyaxares were fighting a battle on that day. Another reliable bit of information is that he did geometrical research, which enabled him to measure the pyramids. However, his most important contribution to European civilization is his attempt to give rational explanations for physical phenomena. Behind the phenomena was not a catalogue of deities, but one single, first principle. Although his identification of this principle with water is rather unfortunate, his idea to look for deeper causes was the true beginning of philosophy and science. Thales died after 547.
	Thales was not the only one who was looking for a first cause. Pythagoras of Samos (c.570-c.480) did the same. According to legend, he left his country and studied with the wise men of Egypt, but was taken captive when the Persian king Cambyses invaded the country of the Nile (525). He now became a student of the Chaldaeans of Babylon and the Magians of Persia. Some even say that he visited the Indian Brahmans, because Pythagoras believed in reincarnation. At the end of the sixth century, he lived in southern Italy, where he founded a community of philosophers. In his view, our world was governed by numbers, and therefore essentially harmonious.
	Heraclitus was a rich man from Ephesus and lived c.500, during the Persian occupation of his home town. His philosophical work consists of a series of cryptical pronouncements that force a reader to think. Unfortunately, a great part of his work is lost, which makes it very difficult to reconstruct Heraclitus' ideas. It seems certain, however, that he thought that the basic principle of the universe was the logos, i.e. the fact that it was rationally organized and therefore understandable. Bipolar oppositions are one form of organization, but the sage understands that these oppositions are just aspects of one reality. Fire is the physical aspect of the perfect logos.
	Parmenides of Elea was a younger contemporary of Heraclitus, but he lived at the other end of the Greek world: in Italy. Both men were intrigued by the immense variety of phenomena, but where Heraclitus discerned order in the chaos, Parmenides pointed out that the endless variety and eternal changes were just an illusion. In a long poem, which partially survives, he opposed 'being' to 'not being', and pointed out that change was impossible, because it would mean that something that was 'not being' changed into 'being', which is absurd. In other words, we had to distrust our senses and rely solely on our intellect. The result was a distinction between two worlds: the unreal world which we experience every day, and the reality, which we can reach by thinking. This idea was to prove one of the most influential in western culture.
	One of the solutions to the problem postulated by Parmenides, was the hypothesis of Democritus of Abdera: matter is made up from atoms. There was no real evidence for this idea (which was not completely new), but it explained why change was possible. The atoms were always moving and clustering in various, temporary combinations. Therefore, things seemed to change, but 'not being' never changed into 'being'. (It was assumed that 'not being' was a vacuum, which means that it is in fact not a 'not being' because a vacuum exists in four dimensions.) The consequence of this idea is that we are allowed to use our senses, although Democritus warns us to be careful.
	Thales, Pythagoras, Heraclitus, Parmenides, and Democritus had been trying to explain the diversity of nature. The object of the philosophy of the Athenian Socrates (469-399) was altogether different: he was interested in ethics. It was his axiom that no one would knowingly do a bad thing. So knowledge was important, because it resulted in good behavior. If we are to believe his student Plato, Socrates was always asking people about what they knew, and invariably they had to admit that they did not really understand what was meant by words like courage, friendship, love etc. Socrates was never without critics. The comic poet Aristophanes ridiculed him in The clouds, and when his pupil Alcibiades had committed high treason, Socrates' position became very difficult. He was forced to drink hemlock after a charge that he had corrupted the youth. Among his students were Antisthenes, Plato and Xenophon.
	In the decade after the death of Socrates, Antisthenes (c.445-c.365) was the most important Athenian philosopher. Like his master, he tried to find out what words mean, but he was convinced that it was not possible to establish really good definitions (which brought him into conflict with Plato). He did only partially agree with Socrates that someone who knew what was good, would not do a bad thing. Antisthenes added that one also had to be strong enough ("as strong as Socrates") to pursue what was good. Therefore, Antisthenes recommended physical training of all kinds, and wanted his students to refrain from luxury. His most famous pupil was Diogenes of Sinope.
	The Athenian philosopher Plato (427-347) is usually called a pupil of Socrates, but his ideas are no less inspired by Parmenides. Plato accepted the world of the phenomena as a mere shadow of the real world of the ideas. When we observe a horse, we recognize what it is because our soul remembers the idea of the horse from the time before our birth. In Plato's political philosophy, only wise men who understand the dual nature of reality are to rule the country. He made three voyages to Syracuse to establish his ideal state, both times without lasting results. Plato's hypothesis that our soul was once in a better place, and mankind lives in a fallen world, made it easy to combine platonic philosophy and Christianity, which accounts for the popularity of Platonism in Late Antiquity. One element, however, was not acceptable: the idea of platonic love - a homosexual relation with pedagofical aspects. Christianity could not accept this, but modern Athens boasts a nightclub called Plato. The philosopher might have appreciated the irony.
	Diogenes of Sinope (c.412-c.323) was a student of Antisthenes. Both men are called the founder of the school that is known as Cynicism. The essential point in this world-view is that man suffers from too much civilization. We are happiest when our life is simplest, which means that we have to live in accordance with nature - just like animals. Human culture, however, is dominated by things that prevent simplicity: money, for example, and our longing for status. Like his master, Diogenes refrained from luxury and often ridiculed civilized life. His philosophy gained some popularity because he focused upon personal integrity, whereas men like Plato and Aristotle had been thinking about man's life and honor as member of a city state - a type of political unit that was losing importance in the age of Alexander the Great. However, we can not return to nature. The Cynics became some sort of jesters, accepted at the royal courts because their criticism was essentially harmless.
	Plato's most famous student was the Macedonian scientist Aristotle of Stagira (384-322). After the death of his master, he studied biology and accepted a position as teacher of the Macedonian crown prince Alexander at Mieza. When the Macedonians subdued Greece, Aristotle founded a school at Athens. Most of his writings are lost; what remains are his lecture notes, which were rediscovered in the first century BCE. During the last decades, scholars have started to re-examine the fragments of the lost works, which has led to important changes in our understanding of Aristotle's philosophy. However, the accepted view remains that he replaced his master's speculations with a more down-to-earth philosophy. His main works are the Prior Analytics (in which he described the rules of logic), the Physics, the Animal History, the Rhetorics, the Poetics, the Metaphysics, the Nicomachean Ethics, and the Politics. All these books have become classics, and it is not exaggerated to say that Aristotle is the most influential philosopher of all ages and the founder of modern science.
	All philosophers are confident that rational thinking is the road to truth. Except for Pyrrho of Elis (c.360-c.270BCE), who entertained some doubts about the quest for knowledge. He argued that we can not fully comprehend nature, do not know for certain whether a statement is true or false, and are unable to build an ethical system on so weak a fundament. People would be happier if they gave up these useless intellectual exercises and postponed their judgment. The result was a conservative political philosophy, because Pyrrho recommended that, even though we had no moral absolutes, we should live by time-honored traditions. The weakness of his system is, of course, twofold: in the first place, one can not postpone a judgment forever, because sometimes action has to be undertaken; in the second place, how can you be certain that certain knowledge is impossible? Pyrrho's world-view is called Skepticism, and may be compared to the postmodernist philosophy of the 1980's.
	We live happiest when we are free from the pains of life, and a virtuous life is the best way to obtain this goal. This is, in a nutshell, the view of the Samian philosopher Epicurus (342-271). In his opinion, we are unable to understand the gods, who may or may not have created this world but are in any case not really interested in mankind. Nor do we know life after death - if there is an existence at all after our bodies have decomposed. Therefore, we must not speculate about gods and afterlife. In Antiquity, Epicurism was the most popular of all philosophical schools, a popularity which it partially owed to the fact that its founder had explained his thoughts in several maxims, which even the illiterate could remember. Predictably, Christian philosophers attacked Epicurus' ideas about the afterlife and divine providence.
	After the conquests of Alexander, the world was larger than ever, and the city-state had ceased to be an important political unit. Like Diogenes and Epicurus, Zeno of Citium (336-264 BCE) ignored traditional values like prestige and honor, and focused on man's inner peace. In his view, this was reached when a person accepted life as it was, knowing that the world was rationally organized by the logos. A man's mind should control his emotions and body, so that one could live according to the rational principles of the world. It has often been said that Zeno's ideas combine Greek philosophy with Semitic mysticism, but except for his descent from a Phoenician town on Cyprus, there is no proof for this idea. This philosophy, called Stoicism, became very influential under Roman officials.
	Zeno was succeeded as head of the Stoic school at Athens by Cleanthes, who was in turn succeeded by Chrysippus, a native of Soli in Cilicia (c.279-c.206). His contributions to the development of philosophy can especially be found in the field of logic, where he studied paradoxes and the way an argument should be constructed. He also reflected upon the use of allegoresis, which is a way to read a text metaphorically and find hidden meanings (or construct them). From now on, philosophers started to use the epics of Homer and the tragedies of Euripides as if they were philosophical treatises. Finally, Chrysippus was the man who concluded that if the rational principle of the universe, the logos, was divine, the world could be defined as a manifestation of God.
	We are ill-informed about the development of philosophy after the origin of the Stoa, Epicurism, Skepticism, Cynicism, Aristoteleanism, and Platonism. For several reasons, nearly all texts are lost. This was also the fate of the works of the Stoic sage Posidonius of Apamea (c.135-51), but his books are often quoted by other authors. As a philosopher, he was not an innovator, but applied the theory to science and scholarship. For example, his Histories were a philosophical continuation of the World History of Polybius. Among his other publications were treatises in which the Stoic world view was applied to everyday subjects: On anger, On virtue, and Consolation. Being more interested in educating the masses than in theoretical purity, he often borrowed ideas from other schools. Philosophy after Posidonius often was a cross-fertilization between viewpoints (e.g., Plutarch and Plotinus).
	The charismatic teacher and miracle worker Apollonius lived in the first century AD. He was born in Tyana and gave a new interpretation to Pythagoreanism, which was essentially a combination of ascesis and mysticism. In his books On astrology and On sacrifices, he demanded bloodless offerings to the One God, who needs nothing even from beings higher than ourselves. This brought Apollonius into conflict with the religious establishment, but he was recognized as a great sage and received divine honors in the third century. Although the Athenian Philostratus wrote a lengthy Life of Apollonius, hardly anything is certain about the man who was and is frequently compared to the Jewish sage and miracle worker Jesus of Nazareth.
	In his own age, the Delphic oracle priest Plutarch of Chaeronea (46-c.122) was immensely popular because he was, like Posidonius, able to explain philosophical discussions to a general audience. Among his Moral treatises are treatises like Checking anger, the useful The art of listening, the fascinating How to know whether one progresses to virtue, and the charming Advice to bride and groom. Plutarch also wrote double biographies, in which he usually compared a Greek to a Roman (e.g., Alexander and Julius Caesar). In the epilogue, he analyzed their respective characters. The result is not only an entertaining biography, but also a better understanding of a morally exemplary person, which the reader can use for his own progress to virtue.
	Born in Phrygia, Epictetus (c.50-c.125 CE) became a slave of the emperor Nero's courtier Epaphroditus. When he was old, useless and therefore "freed" from slavery, he had to make a living and started to teach the Stoic philosophy, first at Rome and (after the emperor Domitian had expelled the philosophers in 89) at Nicopolis in western Greece. Because Epictetus was able to explain Stoicism in a systematic way and with an open eye to its practical applications, he had many students from the rich senatorial order, which ruled the Roman empire. Among these men were the future emperor Hadrian and the historian Arrian of Nicomedia, who published several of his conversations. Epictetus wrote a Handbook, which is arguably the most popular book on philosophy that was ever written.
	After the age of Posidonius, it was not uncommon that philosophers from one school borrowed concepts and ideas from other branches of philosophy. Slowly, the schools were merging, and a new synthesis (called Neo-Platonism) was created by Plotinus (205-270). Like Plato, he accepted that our world was a mere shadow of the world of the ideas, which was in turn -and this was a novel idea- a shadow of an even higher world, which was again a shadow of the One God. In other words, the world has four levels of reality: God was the highest level, and then there were the levels of the intellect, the soul, and matter. (That matter is more real than the speculative levels of existence, was an unusual idea in Antiquity.)  According to Plotinus, the wise man would try, by means of ascesis, to free his soul from matter and unite it with God. Plotinus achieved this mystical unity several times. His philosophy was adopted by the fathers of the church Ambrose and Augustine, and was to remain the philosophical school par excellence until Aristotle was rediscovered in the twelfth century.