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.

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.

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.

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."

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.

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.

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.

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).

Long before historians added "the Great" to his name, Alexander stated, "I would rather live a short life of glory than a long one of obscurity." He certainly lived up to this ambition, conquering most of the known world in just 13 years before his death at age 33. Though undefeated in his lifetime, Alexander died unsatisfied - he always wanted more. His ambitious personality comes as no surprise considering his upbringing: born to powerful parents, he studied under Aristotle and grew up believing himself a descendant of the Greek heroes Achilles and Heracles.

Josh Kaufman founded PersonalMBA.com as an alternative to the business school boondoggle. His blog has introduced hundreds of thousands of readers to the best business books and most powerful business concepts of all time. Now, he shares the essentials of entrepreneurship, marketing, sales, negotiation, operations, productivity, systems design, and much more, in one comprehensive volume. The Personal MBA distills the most valuable business lessons into simple, memorable mental models that can be applied to real-world challenges.

Durant lucidly describes the philosophical systems of such world-famous “monarchs of the mind” as Plato, Aristotle, Francis Bacon, Spinoza, Kant, Voltaire, and Nietzsche. Along with their ideas, he offers their flesh-and-blood biographies, placing their thoughts within their own time and place and elucidating their influence on our modern intellectual heritage. This book is packed with wisdom and wit.

From two leaders in executive education at Harvard Business School, here are the mental habits and proven strategies you need to achieve outstanding results in any negotiation.

Economic forces are everywhere around you. But that doesn't mean you need to passively accept whatever outcome those forces might press upon you. Instead, with these 12 fast-moving and crystal clear lectures, you can learn how to use a small handful of basic nuts-and-bolts principles to turn those same forces to your own advantage.

Today's successful careers are not planned out in advance. They develop when people are prepared for opportunities because they have asked themselves the right questions and have rigorously assessed their unique characteristics. This article challenges listeners to take responsibility for managing their futures, both in and out of the office.

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.

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.

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.

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.

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).

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.

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.

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.