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Plato witnessed the death of Socrates at the hands of the Athenian democracy in 399 BC. Perhaps fearing for his own safety, he left Athens temporarily and traveled to Italy, Sicily, and Egypt. In 387 Plato founded the Academy in Athens, the institution often described as the first European university. It provided a comprehensive curriculum, including such subjects as astronomy, biology, mathematics, political theory, and philosophy. Aristotle was the Academy's most prominent student. Pursuing an opportunity to combine philosophy and practical politics, Plato went to Sicily in 367 to tutor the new ruler of Syracuse, Dionysius the Younger, in the art of philosophical rule. The experiment failed. 

Plato made another trip to Syracuse in 361, but again his engagement in Sicilian affairs met with little success. The concluding years of his life were spent lecturing at the Academy and writing. He died at about the age of 80 in Athens in 348 or 347 BC. Works Plato's writings were in dialogue form; philosophical ideas were advanced, discussed, and criticized in the context of a conversation or debate involving two or more persons. The earliest collection of Plato's work includes 35 dialogues and 13 letters. The authenticity of a few of the dialogues and most of the letters has been disputed. 

Early Dialogues

The dialogues may be divided into early, middle, and later periods of composition. The earliest represent Plato's attempt to communicate the philosophy and dialectical style of Socrates.

Several of these dialogues take the same form. Socrates, encountering someone who claims to know much, professes to be ignorant and seeks assistance from the one who knows. As Socrates begins to raise questions, however, it becomes clear that the one reputed to be wise really does not know what he claims to know, and Socrates emerges as the wiser one because he at least knows that he does not know. Such knowledge, of course, is the beginning of wisdom. Included in this group of dialogues are Charmides (an attempt to define temperance), Lysis (a discussion of friendship), Laches (a pursuit of the meaning of courage), Protagoras (a defense of the thesis that virtue is knowledge and can be taught), Euthyphro (a consideration of the nature of piety), and Book I of the Republic (a discussion of justice). 

Middle and Late Dialogues 

The dialogues of the middle and later periods of Plato's life reflect his own philosophical development. The ideas in these works are attributed by most scholars to Plato himself, although Socrates continues to be the main character in many of the dialogues. The writings of the middle period include Gorgias (a consideration of several ethical questions), Meno (a discussion of the nature of knowledge), the Apology (Socrates' defense of himself at his trial against the charges of atheism and corrupting Athenian youth), Crito (Socrates' defense of obedience to the laws of the state), Phaedo (the death scene of Socrates, in which he discusses the theory of Forms, the nature of the soul, and the question of immortality), the Symposium (Plato's outstanding dramatic achievement, which contains several speeches on beauty and love), the Republic (Plato's supreme philosophical achievement, which is a detailed discussion of the nature of justice). 

The works of the later period include the Theaetetus (a denial that knowledge is to be identified with sense perception), Parmenides (a critical evaluation of the theory of Forms), Sophist (further consideration of the theory of Ideas, or Forms), Philebus (a discussion of the relationship between pleasure and the good), Timaeus (Plato's views on natural science and cosmology), and the Laws (a more practical analysis of political and social issues). 

Theory of Forms

At the heart of Plato's philosophy is his theory of Forms, or Ideas. Ultimately, his view of knowledge, his ethical theory, his psychology, his concept of the state, and his perspective on art must be understood in terms of this theory. Theory of Knowledge Plato's theory of Forms and his theory of knowledge are so interrelated that they must be discussed together. Influenced by Socrates, Plato was convinced that knowledge is attainable. He was also convinced of two essential characteristics of knowledge.

First, knowledge must be certain and infallible. Second, knowledge must have as its object that which is genuinely real as contrasted with that which is an appearance only. Because that which is fully real must, for Plato, be fixed, permanent, and unchanging, he identified the real with the ideal realm of being as opposed to the physical world of becoming. One consequence of this view was Plato's rejection of empiricism, the claim that knowledge is derived from sense experience. He thought that propositions derived from sense experience have, at most, a degree of probability. 

They are not certain. Furthermore, the objects of sense experience are changeable phenomena of the physical world. Hence, objects of sense experience are not proper objects of knowledge. Plato's own theory of knowledge is found in the Republic, particularly in his discussion of the image of the divided line and the myth of the cave. In the former, Plato distinguishes between two levels of awareness: opinion and knowledge. Claims or assertions about the physical or visible world, including both commonsense observations and the propositions of science, are opinions only. Some of these opinions are well founded; some are not; but none of them counts as genuine knowledge.

The higher level of awareness is knowledge, because there reason, rather than sense experience, is involved. Reason, properly used, results in intellectual insights that are certain, and the objects of these rational insights are the abiding universals, the eternal Forms or substances that constitute the real world. The myth of the cave describes individuals chained deep within the recesses of a cave. Bound so that vision is restricted, they cannot see one another. The only thing visible is the wall of the cave upon which appear shadows cast by models or statues of animals and objects that are passed before a brightly burning fire. Breaking free, one of the individuals escapes from the cave into the light of day. With the aid of the sun, that person sees for the first time the real world and returns to the cave with the message that the only things they have seen heretofore are shadows and appearances and that the real world awaits them if they are willing to struggle free of their bonds. 

The shadowy environment of the cave symbolizes for Plato the physical world of appearances. Escape into the sun-filled setting outside the cave symbolizes the transition to the real world, the world of full and perfect being, the world of Forms, which is the proper object of knowledge. 

Nature of Forms

The theory of Forms may best be understood in terms of mathematical entities. A circle, for instance, is defined as a plane figure composed of a series of points, all of which are equidistant from a given point. No one has ever actually seen such a figure, however. What people have actually seen are drawn figures that are more or less close approximations of the ideal circle. In fact, when mathematicians define a circle, the points referred to are not spatial points at all; they are logical points. They do not occupy space. Nevertheless, although the Form of a circle has never been seen-indeed, could never be seen-mathematicians and others do in fact know what a circle is. That they can define a circle is evidence that they know what it is. 

For Plato, therefore, the Form "circularity" exists, but not in the physical world of space and time. It exists as a changeless object in the world of Forms or Ideas, which can be known only by reason. Forms have greater reality than objects in the physical world both because of their perfection and stability and because they are models, resemblance to which gives ordinary physical objects whatever reality they have. Circularity, squareness, and triangularity are excellent examples, then, of what Plato meant by Forms. An object existing in the physical world may be called a circle or a square or a triangle only to the extent that it resembles ("participates in" is Plato's phrase) the Form "circularity" or "squareness" or "triangularity." Plato extended his theory beyond the realm of mathematics. Indeed, he was most interested in its application in the field of social ethics. 

The theory was his way of explaining how the same universal term can refer to so many particular things or events. The word justice, for example, can be applied to hundreds of particular acts because these acts have something in common, namely, their resemblance to, or participation in, the Form "justice." An individual is human to the extent that he or she resembles or participates in the Form "humanness." If "humanness" is defined in terms of being a rational animal, then an individual is human to the extent that he or she is rational. A particular act is courageous or cowardly to the extent that it participates in its Form. An object is beautiful to the extent that it participates in the Idea, or Form, of beauty. Everything in the world of space and time is what it is by virtue of its resemblance to, or participation in, its universal Form. 

The ability to define the universal term is evidence that one has grasped the Form to which that universal refers. Plato conceived the Forms as arranged hierarchically; the supreme Form is the Form of the Good, which, like the sun in the myth of the cave, illuminates all the other Ideas. There is a sense in which the Form of the Good represents Plato's movement in the direction of an ultimate principle of explanation. Ultimately, the theory of Forms is intended to explain how one comes to know and also how things have come to be as they are. In philosophical language, Plato's theory of Forms is both an epistemological (theory of knowledge) and an ontological (theory of being) thesis. 

Political Theory 

The Republic, Plato's major political work, is concerned with the question of justice and therefore with the questions "what is a just state" and "who is a just individual?" The ideal state, according to Plato, is composed of three classes. The economic structure of the state is maintained by the merchant class. Security needs are met by the military class, and political leadership is provided by the philosopher-kings. A particular person's class is determined by an educational process that begins at birth and proceeds until that person has reached the maximum level of education compatible with interest and ability. Those who complete the entire educational process become philosopher-kings. 

They are the ones whose minds have been so developed that they are able to grasp the Forms and, therefore, to make the wisest decisions. Indeed, Plato's ideal educational system is primarily structured so as to produce philosopher-kings. Plato associates the traditional Greek virtues with the class structure of the ideal state. Temperance is the unique virtue of the artisan class; courage is the virtue peculiar to the military class; and wisdom characterizes the rulers. Justice, the fourth virtue, characterizes society as a whole. The just state is one in which each class performs its own function well without infringing on the activities of the other classes. Plato divides the human soul into three parts: the rational part, the will, and the appetites. 

The just person is the one in whom the rational element, supported by the will, controls the appetites. An obvious analogy exists here with the threefold class structure of the state, in which the enlightened philosopher-kings, supported by the soldiers, govern the rest of society.

Ethics 

Plato's ethical theory rests on the assumption that virtue is knowledge and can be taught, which has to be understood in terms of his theory of Forms. As indicated previously, the ultimate Form for Plato is the Form of the Good, and knowledge of this Form is the source of guidance in moral decision making. Plato also argued that to know the good is to do the good. The corollary of this is that anyone who behaves immorally does so out of ignorance. This conclusion follows from Plato's conviction that the moral person is the truly happy person, and because individuals always desire their own happiness, they always desire to do that which is moral.

Art 

Plato had an essentially antagonistic view of art and the artist, although he approved of certain religious and moralistic kinds of art. Again, his approach is related to his theory of Forms. A beautiful flower, for example, is a copy or imitation of the universal Forms "flowerness" and "beauty." The physical flower is one step removed from reality, that is, the Forms. A picture of the flower is, therefore, two steps removed from reality. This also meant that the artist is two steps removed from knowledge, and, indeed, Plato's frequent criticism of the artists is that they lack genuine knowledge of what they are doing. Artistic creation, Plato observed, seems to be rooted in a kind of inspired madness. Influence Plato's influence throughout the history of philosophy has been monumental.

When he died, Speusippus became head of the Academy. The school continued in existence until AD 529, when it was closed by the Byzantine emperor Justinian I, who objected to its pagan teachings. Plato's impact on Jewish thought is apparent in the work of the 1st-century Alexandrian philosopher Philo Judaeus. Neoplatonism, founded by the 3rd-century philosopher Plotinus, was an important later development of Platonism. The theologians Clement of Alexandria, Origen, and St. Augustine were early Christian exponents of a Platonic perspective. Platonic ideas have had a crucial role in the development of Christian theology and also in medieval Islamic thought (see Islam). 

During the Renaissance, the primary focus of Platonic influence was the Florentine Academy, founded in the 15th century near Florence. Under the leadership of Marsilio Ficino, members of the Academy studied Plato in the original Greek. In England, Platonism was revived in the 17th century by Ralph Cudworth and others who became known as the Cambridge Platonists. Plato's influence has been extended into the 20th century by such thinkers as Alfred North Whitehead, who once paid him tribute by describing the history of philosophy as simply "a series of footnotes to Plato." Source: Microsoft Encarta 97