• Never do wrong, nor participate in wrongdoing
• That a person who understood what was good and right would not act against those principles

On numerous occasions, he demonstrated unshakable adherence to these values, and at the expense of his life. After the Battle of Arginusae, a majority in the Athenian assembly demanded death without trial for the admirals. Socrates – who was the assembly’s president – refused to put the proposal to a vote because he felt it was wrong to condemn anyone without a fair trial. Though he was threatened for not doing so, he refused to change his position. Later, after the overthrow of democracy, the so-called Thirty Tyrants ordered him to arrest an innocent citizen. Again, he refused. Though these principled stands won him the admiration of many, it created a great resentment among those in power. Later, when democracy was restored, he was condemned to death. He was accused of impiety and of corrupting the youth of Athens with his ideas. Following Socrates’ death, his influence began to grow considerably in Greek and Roman philosophy. Many of his followers, including Xenophon (430-350 B.C.) and Aeschines (390 B.C.), tried to preserve his philosophical method by writing Socratic dialogues. Schools were also founded to help promote various aspects of his philosophy.

9. Plato (427-347 B.C.)
Plato was the most important follower of Socrates. He is most famous for his book The Republic in which he described the creation of an ideal society. The Republic was a socio-political book that outlined how an ideal society would function. According to Plato, an ideal society would be ruled by an elite, an elite skilled in the art of reason: the philosophers. There would be two other classes within that society: the warriors, who would protect the society from external threats, and the working population. Neither the philosophers nor the warriors would be allowed to own property. They would be expected to lead an austere and simple life, dedicated to the harmony of the ideal state. Despite the influence and importance of The Republic, Plato’s most significant contribution to theoretical philosophy was in his Theory of Forms or Ideas. In questioning someone’s strongly held opinion, Socrates had often asked what it was that people were referring to when they talked about something being good, or beautiful, or brave. Plato had asked what is the Idea or Eidos that a person has in mind when he called something "good." Plato concluded that this Idea or Eidos exists in the world beyond that of our senses. Plato called this the world of Ideas. In this world of Ideas, what we perceive with our senses is but a very imperfect representation of an external and eternal Idea. In one of Plato’s most famous examples, in the seventh book of his Politeia, he created the scenario of the man in the cave. In this example, Plato said that we are like people in a cave who are looking at a wall – and seeing nothing but the shadows of the real things that are behind our backs being projected on the wall. He said the philosopher was the one who had the perceptual tools to leave the cave and see the world – the real world of Ideas. Because the philosophers had the ability to really see the "real" world, then it would follow that they should also be the governing elite who would guide the rest of society.

10. Pythagoras (570-495 B.C.)
In the 6th century, a Greek colony arose in Italy led by Pythagoras of Samos. Unlike many of the earlier philosophers, the Pythagoreans tried to develop a completely new and unique philosophy or their own. Pythagoras – who had traveled in the East and to Egypt – founded a philosophical society in southern Italy. Because the Pythagoreans were a secretive quasi-religious sect, only fragments of their teaching survive. However, scholars believe that he introduced the concept of the "transmigration of souls" because of what he had learned in the East. But Pythagoras’ most influential contribution, both for philosophy and for science, was his doctrine that "all things are numbers," meaning that the "essences" and "structures" of all things can be determined by finding the numerical relations contained in them. Pythagoras is most recognized for his contribution to mathematics. He discovered that mathematical ratios exist in the real world. For instance, the length of a musical string directly affects the pitch of that string when plucked. Pythagoras also conducted an extensive study of the motion of celestial objects. In both of these previous examples, he recognized that there was a fundamental order to the nature of things. He concluded that the aim of human life was to life in harmony with this natural order.

11. Aristotle (384-322 B.C.)
Aristotle, a contemporary of Plato, soon raised objections to the theory of Ideas put forward by Plato. Aristotle declared that it is unnecessary to assume that there is a separate realm of perfect Ideas. He rejected Plato’s claim that we are but imperfect copies of perfect Ideas. Aristotle recognized that all living things develop from an imperfect state to a more perfected state. A seed grows into a plant; a baby grows into an adult. The question for Aristotle, then, was what kind of perfection was it possible for a human being to reach. Aristotle was a great Empiricist, and tried to base his arguments solely on empirical observation. In the 1st century, Aristotle’s writings were rediscovered. Many great schools of commentary and criticism arose to discuss and explain his philosophy. He work was to have a great influence on medieval philosophy.

12. Hellenistic and Roman Philosophy
Following the death of Aristotle, the Greek city-states began to crumble. The Hellenistic kings who had succeeded Alexander made life more troubled and insecure. It was in this troubled atmosphere that two dogmatic philosophical systems were created: the Stoic and the Epicurean. Each in their own way gave consolation and fortitude in difficult times.

Stoicism
Zeno of Citium (300 B.C.) founded the Stoic system of philosophy. The system comprised many of the elements found in both the philosophies of Socrates and Heraclitus. It stressed the importance of endurance and self-control. The Stoics taught that pleasure and pain were of no importance to a person’s happiness. The possession of virtue, and not worldly wealth or power, was of prime importance. The Stoics also believed in divine providence.

Epicureanism
The Epicureans took the opposite approach to that of the Stoics. They held that pleasure was the very essence of a happy life, and that the gods were indifferent to human beings. But Epicurus wasn’t an atheist. Lucretius (94-55 B.C.) praised him for liberating mankind from its religious fears. Epicurus felt it was important to look upon the gods as perfect beings, in order that men could also approach perfection.

13. The Skeptics
Pyrrho of Elis (365-270 B.C.) founded the school of Skepticism. The main doctrine of this philosophy was that we could never know anything with certainty. We cannot even trust our own senses. This philosophy had a profound influence on later philosophers such as Hume and Kant.

14. Medieval Philosophy
The period from the 5th century to the 15th century is known as the Middle Ages, or the medieval period. Medieval literally means "the in-between time" which began in the 5th century with the fall of the Roman Empire, and ended in the 15th century with the beginning of the Renaissance. During the medieval period, western thinkers sought to create a synthesis between religion and philosophy. Early medieval philosophy drew heavily upon the neoplatonic philosophy of Plotinus, who seemed to provide the most support for religious belief. Later in the medieval period, the metaphysics of Aristotle gained a wider audience. In every instance, the goal was to create a philosophical foundation for religious thought. The philosophers of the medieval period were usually churchmen. Their goal was to use philosophy as a means of giving a rational interpretation of Christian faith. Any philosophical speculation, other than supporting church dogma, was strictly discouraged. Philosophy during this period was considered to be the "handmaiden" of theology, and philosophical speculation other than supporting the church was frowned upon. Despite these constraints, much creative philosophy did take place during this period. With the collapse of the Roman Empire in the 5th century, and the decline and fall of its civilization, a new Christian culture emerged in Europe. Monasteries now became the new centers of learning and education. During this period, philosophers such as Augustine (354-430), Boethius (480-525), and Anselm (1033-1109) attempted to unite Greek thought with Christian belief.

15. Saint Augustine / Augustine of Hippo (354-430)
Saint Augustine was the first great medieval philosopher. Augustine was a North African rhetorician - and follower of Manichaeanism – who converted to Christianity. He devoted his career towards creating a philosophical system that employed aspects of neoplatonic thinking in support of Christian orthodoxy. Augustine believed that philosophy was only useful or meaningful to those who already had faith. He said, "I believe in order that I may understand." He rejected the epistemological criticisms of earlier philosophers such as the Skeptics. He argued that even if everything around us is an illusion, then one inescapable truth will still remain: that I exist. This view has much in common with Descartes. Augustine, for example, believed that there existed – beyond the world of the senses – a spiritual and eternal realm of truth. This truth is the object of the human mind, and the goal of all our striving. He identified this truth with the God of the Christian faith. Augustine felt that man was a combination of two substances: the body and the soul. The soul is the most important and the superior of the two. The truth of existence could not lie in a contingent and changing world, but in the truth that could only be found by inner reflection. The mind was the means of finding the "intelligible light" where the real truth could be found. Consequently, the soul’s immortality is proved by its possession of this unchanging "truth." Thus, the "intelligible light" is where real truth is found. Augustine’s moral philosophy was also to have a profound effect on the development of Christian theology. Augustine argued that evil may exist, but it isn’t real. Evil is, by his definition, the absence of good. Augustine believed that we were all sinful by nature, but that a good God had given us the freedom to choose our own actions. But the question then arises: if we are sinful by nature – and have original sin – then how can we make free and moral choices? Augustine argued that the redemptive grace of God alone offers us hope. To prove that God exists, Augustine drew heavily upon the ideas of Plato and Pythagoras. If we are capable of achieving mathematical knowledge - thereby transcending the sensory realm of appearance - we can therefore logically conclude that our souls are immaterial and immortal. Augustine recognized that additional metaphysical support was needed for this argument. Where does the abstract mathematical knowledge come from? What is the eternal source of these abstractions? This, he concluded, must be God.

16. Boethius / Anicius Manlius Severinus (480-525)
Boethius was one of the most important philosophers of the Middle Ages, not just for his creative work, but also for the fact that he translated many Greek works into Latin. He translated the logical writings of Porphyry (232-304) – a Neoplatonist – and also many of the works of Aristotle. In his creative philosophy, his presentation of the Aristotelian doctrine of universals was very influential for later philosophers. Are "universals" real? If so, are they corporeal or incorporeal? If incorporeal, do they exist in the world of our senses, or apart from it? If "universals" are not real, are they then only mental concepts? These questions were to become a dominant focus of medieval philosophy.