Schopenhauer, the "philosopher of pessimism", makes it very plain that he regards the world and our life in it as a bad joke. But if the world is indifferent to our fate, it doesn't thwart us on purpose. The world's facade is supported by what Schopenhauer calls the Universal Will, blind and without purpose. This Will brings on all our misery and suffering; our only hope is to liberate ourselves from its power and from the trappings of individualism and egoism that are at its mercy.

Just a century after it had begun, philosophy entered its greatest age with the appearance of Socrates, who spent so much of his time talking about philosophy on the streets of Athens that he never got around to writing anything down. His method of aggressive questioning, called dialectic, was the forerunner of logic; he used it to cut through the twaddle of his adversaries and arrive at the truth. Rather than questioning the world, he believed, we would be better off questioning ourselves.

Immanuel Kant taught and wrote prolifically about physical geography yet never traveled further than forty miles from his home in Kvnigsberg. How appropriate it is then that in his philosophy he should deny that all knowledge was derived from experience. He insisted that all experience must conform to knowledge. According to Kant, space and time are subjective; along with various "categories," they help us to see the phenomena of the world, though never its true reality.

Spinoza's brilliant metaphysical system was derived neither from reality nor experience. Starting from basic assumptions, with a series of geometric proofs he built a universe which was also God, one and the same thing, the classic example of pantheism. Although his system seems an oddity today, Spinoza's conclusions are deeply in accord with modern thought, from science (the holistic ethics of today's ecologists) to politics (the idea that the state exists to protect the individual).

"If we accept Wittgenstein's word for it," Paul Strathern writes, "he is the last philosopher. In his view, philosophy in the traditional sense was finished."

Confucius knew all about life and told us how to behave, but we can't find out precisely what he was up to. His well-meaning platitudes, quaint maxims, and quasi-enigmatic anecdotes combined to produce an ideal philosophy for civil servants. It would appear that his aim was to turn his pupils into good government officials, but his teachings succeeded beyond his wildest expectations, providing rules of conduct and spiritual fodder for more than two thousand years.

We see our age as the greatest in human history, filled with seemingly unending originality. Yet such dynamism is not a necessary characteristic of great eras. Among the most long-lasting and stable civilizations was that of medieval Europe. There stasis was achieved, and with it a stability that permitted the development of structured thought and intellectual embellishment of unparalleled degree.

With Hegel, philosophy became very difficult indeed. His dialectical method produced the most grandiose metaphysical system known to man. Even Hegel conceded that "only one man understands me, and even he does not." Hegel's system included absolutely everything, but its most vital element was the dialectic of the thesis, antithesis, and synthesis. This method sprang from Hegel's ambition to overcome the deficiencies of logic and ascended toward mind as the ultimate reality.

With Friedrich Nietzsche, philosophy was dangerous not only for philosophers but for everyone. His ideas presaged a collective madness that had horrific consequences in Europe in the early 1900s. Though his philosophy is more one of aphorisms than a system, it is brilliant, persuasive, and incisive. His major concept is the will to power, which he saw as the basic impulse for all our acts. Christianity he saw as a subtle perversion of this concept, thus Nietzsche's famous pronouncement, "God is dead."

Rene Descartes spent most of his childhood in solitude, a situation that also came to characterize his adult life. Fortunately, these countless lonely hours helped Descartes produce the declaration that changed all philosophy: "I think, therefore I am." Eventually convincing himself to doubt and disregard sensory knowledge, Descartes found he could prove his existence through his thoughts. This internal information, he believed, was the true reality and external forces were hopelessly deceiving.

Aristotle wrote on everything from the shape of seashells to sterility, from speculations on the nature of the soul to meteorology, poetry, art, and even the interpretation of dreams. Apart from mathematics, he transformed every field of knowledge that he touched. Above all, Aristotle is credited with the founding of logic. When he first divided human knowledge into separate categories, he enabled our understanding of the world to develop in a systematic fashion.

In an age when philosophers had scarcely glimpsed the horizons of the mind, a boy named Aristocles decided to forgo his ambitions as a wrestler. Adopting the nickname Plato, he embarked instead on a life in philosophy. In 387 B.C. he founded the Academy, the world's first university, and taught his students that all we see is not reality but merely a reproduction of the true source. And in his famous Republic he described the politics of "the highest form of state."

In Rousseau we encounter a walking ego, naked sensibility. Feeling triumphs over intellectual argument in his works, which are both deeply stirring and deeply inconsistent. Yet while his contemporaries Kant and Hume may have been superior academic philosophers, the sheer power of Rousseau's ideas was unequaled in his time. It was he who encouraged the introduction of both liberty and irrationality into the public domain.

Karl Marx's devastating critique of capitalism, and his proposal of communism as the answer to the failings of the capitalist system, bore their greatest fruits in the twentieth century with the formation of the communist state in the Soviet Union. This great venture has now all but completely failed. Yet the force of the communist belief offered the prospect of "justice on this earth" to countless numbers. And Marx's critique has influenced generations of thinkers who call themselves Marxists.

During his lifetime, Jean-Paul Sartre enjoyed unprecedented popularity for a philosopher, due partly to his role as a spokesman for existentialism at the opportune moment, when this set of ideas filled the spiritual gap left amidst the ruins of World War II. Existentialism was a philosophy of action and showed the ultimate freedom of the individual. In Sartre's hands, it became a revolt against European bourgeois values.