Thales (625?-546 B.C.): One of the first to conceive of the universe in natural terms -- that there are laws of nature that can be known through a process of rational criticism. He is known to have predicted an eclipse and for falling in a water well while looking at the stars. Also known for applying his knowledge of astronomy to weather forecasting, predicting a bumper olive crop, buying up all the olive presses and making a financial killing. He taught that everything was made of water and that the Earth was a flat disk floating on water.

Anaximander (610-ca. 547 B.C.): One of the first to believe that the human species arose from lower animals, specifically from fishlike creatures. Made a sundial, determined accurately the length of the year and seasons, made a map of the known world, and taught that the Earth is not supported and sits at the center of the cosmos. He also believed that the Sun, Moon, and the stars are made of fire seen through moving holes in the celestial globe of the sky, and that all reality evolved from a boundless, nondefinable stuff.

Anaximenes (?): Among the first to suggest that qualitative changes (from a liquid to a gaseous state, for example) can be explained as changes in the density of one stuff. He thought this ultimate stuff to be air. He also believed that the Earth and the planets float, and that the Moon shines by reflecting light from the Sun.

Anaxagoras (500-ca. 428 B.C.): He believed that the Sun and Moon were not gods, that the Moon was made of ordinary matter with hills and valleys, and that the Sun was a red-hot stone. For this he was imprisoned for impiety. He dissected animals, studied the anatomy of the brain, and discovered that fish breathe through their gills. However, he thought the cosmos could be better understood qualitatively rather than quantitatively.

Pythagoras (580-ca. 500 B.C.): He believed that the Earth is a sphere and moved around an invisible central fire (the Altar of Zeus). He is most noted for teaching that mathematical harmonies are the basis for all natural events.

Empedocles (490-ca. 4300 B.C.): He taught that the basic elements were earth, water, air, and fire. He believed that light had a finite speed, but a very great one, and that many more life forms once existed on Earth than we see now. He conducted a famous experiment with a water clock (clepsydra), indicating that air must be made of something. Some accounts claim he thought of himself as a god and died by leaping into hot lava of the volcano Aetna.

Democritus (460-ca. 370 B.C.): Nicknamed the laughing philosopher. Taught that all that exists are atoms and empty space, that worlds such as the Earth evolved and decayed, that there were many others, some inhabited, others not, and that the Milky Way was composed of millions of unresolved stars.

Socrates (470-399 B.C.): Apparently he held very few doctrines, but believed passionately in the fruitfulness of the pursuit of truth, the unlimited growth potential of humankind, and the goodness of the cosmos. Although he was more interested in human behavior than the natural world, he is quoted as saying, "Man must rise above the Earth -- to the top of the clouds and beyond -- for only thus will he fully understand the world in which he lives." He was condemned to death for impiety and corrupting the youth of Athens.

Plato (427-347 B.C.): A student of Socrates. He defended the Greek rationalist tradition against the attack of the sophists. Emphasized the supreme importance of mathematics in gaining knowledge, recognized the limitations of experiment and empirical knowledge, and accepted an operationalist or instrumentalist view of physical science. He established the first school dedicated to the pursuit of universal knowledge, the Academy, and suggested that women have the same intellectual potential as men.

Aristotle (384-322 B.C.): A student of Plato. The world's first systematic biologist. He emphasized the importance of empirical observation, tutored Alexander the Great, and developed a physics of motion that was very influential until the 16th century. As with Plato, his primary interest was to reaffirm the existence of a public and knowable reality which had been questioned by the sophists. He also founded a university, the Lyceum.