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."

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.

David Hume reduced philosophy to ruins: he denied the existence of everything, except our actual perceptions themselves. I alone exist, he argued, and the world is nothing more than part of my consciousness. Yet we know that the world remains, and we go on as before. What Hume expressed was the status of our knowledge about the world, a world in which neither religion nor science is certain.

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.

Kierkegaard wasn't really a philosopher in the academic sense. Yet he produced what many people expect of philosophy. His subject was the individual and his or her existence, the "existing being." In Kierkegaard's view, this purely subjective entity lay beyond the reach of reason, logic, philosophical systems, theology, or even "the pretenses of psychology." Nonetheless, it was the source of all these subjects. The branch of philosophy to which Kierkegaard gave birth has come to be known as existentialism.

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.

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.

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.

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.

One of the two major philosophical traditions of the twentieth century was linguistic analysis, derived largely from Wittgenstein. The other, diametrically opposed, came from Heidegger, and its fundamental question was, "What is the meaning of existence?" For Heidegger, this question could not simply be "analyzed away". It was beyond the reach of logic or reason. It was the primary "given" of every individual life. To confront it, Heidegger needed to develop an entire new form of philosophy.

"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."

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.

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.

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).