Philosophy really took off, though, with Socrates and Plato in the 5th - 4th Century B.C. (often referred to as the Classical or Socratic period of philosophy). Unlike most of the Pre-Socratic philosophers before him, Socrates was more concerned with how people should behave, and so was perhaps the first major philosopher of Ethics. He developed a system of critical reasoning in order to work out how to live properly and to tell the difference between right and wrong. His system, sometimes referred to as the Socratic Method, was to break problems down into a series of questions, the answers to which would gradually distill a solution. Although he was careful to claim not to have all the answers himself, his constant questioning made him many enemies among the authorities of Athens who eventually had him put to death.
Socrates himself never wrote anything down, and what we know of his views comes from the "Dialogues" of his student Plato, perhaps the best known, most widely studied and most influential philosopher of all time. In his writings, Plato blended Ethics, Metaphysics, Political Philosophy and Epistemology (the theory of knowledge and how we can acquire it) into an interconnected and systematic philosophy. He provided the first real opposition to the Materialism of the Pre-Socratics, and he developed doctrines such as Platonic Realism, Essentialism and Idealism, including his important and famous theory of Forms and universals (he believed that the world we perceive around us is composed of mere representations or instances of the pure ideal Forms, which had their own existence elsewhere, an idea known as Platonic Realism). Plato believed that virtue was a kind of knowledge (the knowledge of good and evil) that we need in order to reach the ultimate good, which is the aim of all human desires and actions (a theory known as Eudaimonism). Plato's Political Philosophy was developed mainly in his famous "Republic", where he describes an ideal (though rather grim and anti-democratic) society composed of Workers and Warriors, ruled over by wise Philosopher Kings.
The third in the main trio of classical philosophers was Plato's student Aristotle. He created an even more comprehensive system of philosophy than Plato, encompassing Ethics, Aesthetics, Politics, Metaphysics, Logic and science, and his work influenced almost all later philosophical thinking, particularly those of the Medieval period. Aristotle's system of deductive Logic, with its emphasis on the syllogism (where a conclusion, or synthesis, is inferred from two other premises, the thesis and antithesis), remained the dominant form of Logic until the 19th Century. Unlike Plato, Aristotle held that Form and Matter were inseparable, and cannot exist apart from each other. Although he too believed in a kind of Eudaimonism, Aristotle realized that Ethics is a complex concept and that we cannot always control our own moral environment. He thought that happiness could best be achieved by living a balanced life and avoiding excess by pursuing a golden mean in everything (similar to his formula for political stability through steering a middle course between tyranny and democracy).