On May 28, 585 B.C. a solar eclipse cast Greece temporarily into darkness. There had been solar eclipses in this part of the world before, of course, but this one was unique. What was so special about this eclipse was that it had been predicted. And it is the prediction of this solar eclipse which is widely agreed upon as the birth of science and philosophy (this far back in history the two disciplines are indistinguishable), and the predictor, Thales of Miletus, as the first practitioner of this new discipline.
What does it mean to say that Thales was the first philosopher and scientist? Certainly it does not mean that he was the first person to investigate nature and to ask the questions "how?" and "why?" For almost as long as there have been human beings, there have been attempts to explain the world around us. Even the most ancient cultures asked fundamental questions about the workings of natural phenomena and about our place within the cosmos. What distinguishes Thales' attempt from all those that went before him?
There are two primary characteristics that distinguish Thales from all previous thinkers. First, he separated the natural world from the supernatural world by recognizing that natural events fall into general classes (such as earthquakes, rather than a specific earthquake) and that these general classes of events can be explained by appealing to regular sequences of cause and effect, rather than to random acts on the part of anthropomorphic gods. His explanations, unlike the explanations of all those who went before him, do not attribute any sort of human personality to nature. All the previous attempts to understand the world can be described as "mythologies." To answer the questions "why?" and "how?" earlier investigators personified natural phenomena into gods and goddesses, giving them human psychologies and human motivations. Divine jealousies, love affairs, births and deaths provided the framework for explaining the natural world.
Thales' second achievement was to recognize the superiority of rational argument over all other sorts of authority. In the Greece of Thales' day, it was the poets, such as Homer and Hesiod, who had the authority to give explanations of the world, and this authority did not rest in their impressive use of reason, but in their aesthetic sensibility and their alleged divine inspiration (every poem began with an invocation of the divine muses, who were thought to inspire the poet and to speak through him). Though in some sense these accounts were rational (e.g. Hesiod claimed that mountains were born of the earth and that night was born of day) what made one account superior to another was its greater explanatory power. Thales, on the other hand, recognized that what makes one explanation better than another is how well it fits with, and accounts for, the evidence.
Thales, then, was the first to react against the poetic, mythological tradition that (1) attempted to explain nature by projecting human personalities onto it, and (2) appealed to divine authority and literary merit rather than to rationality. Like the poets of his age, Thales wanted to give an account of the origins of the universe (a cosmogony) and to explain the phenomena he observed around him. But unlike all those who had gone before him, he wanted to give this account in natural, rational terms. His was an attempt to discover a real lawfulness within nature, and so observation and prediction become of crucial importance: observation because it reveals the patterns within nature, prediction because it confirms that the patterns thus deduced are correct. In this light, we can see why Thales' prediction of the solar eclipse of 585 B.C. is commonly viewed as the birth of science. What enabled Thales to make this prediction was his newfangled belief in the inherent lawfulness in nature, the belief that natural phenomena follow certain fixed patterns of cause and effect.
It is easy to overlook the enormity of this achievement; when we turn to the content of Thales' philosophy it sounds downright laughable (his fundamental tenet, after all, is that everything in the world is really water). Modern students are tempted to dismiss this hypothesis as a ridiculous mistake and to overlook the enormous cognitive advance that it represents. But Thales' water- centric theory, however off target, is at least trying to give the right sort of explanation, an explanation that treats nature as lawful and as wholly separate from the supernatural, and that at the very least uses rational argument to back up all claims.
For a while, this new method of investigating the world stayed rather localized. The second naturalist to appear on the Greek scene was Thales' student Anaximander, and the third was Anamximander's student, Anaximenes. These three earliest philosophers are commonly called the Milesians, because they all lived in the city of Miletus in Ionia Greece. Together they set up a pattern for philosophy, establishing three questions as fundamental: (1) What is the basic substance of the universe? (2) How does this basic substance give rise to the plurality of objects we observe around us? (3) How does the world maintain itself?
Within less than a half century, though, the Milesian revolution began spreading to other parts of the Mediterranean. Phusikoi (the term Aristotle used to refer to the Presocratics) began to pop up in Italy and other parts of Greece, asking these same three questions and attempting to answer them without appealing to gods and goddesses. Among these, the most important were Xenophanes in Colophon, Greece; Heraclitus in Ephesus, Greece; and Parmenides in Elea, Southern Italy. These phusikoi were reacting to one another, engaging in rational argument and analysis of empirical data. They were continuing the scientific tradition set by the Milesians but also building on it, adding new questions (such as epistemological questions about the limits of human knowledge and metaphysical questions about the possibility of change and motion) and new demands for ever more rigorous reasoning.
After Thales' breakthrough, the next mini-revolution came when Parmenides of Elea, a student of Xenophanes', argued for the impossibility of change in the natural world. He purported to show, based solely on rigorous rational argument, that our observations do not correlate with reality: where we think we see a world full of change, motion, generation, destruction, and a plurality of objects, there is really only one unified, unchanging, eternal, unmoving reality. While the specifics of this claim now seem patently false, the principle behind it—that the world as it really is can be very different from the world as it appears to be—has been validated again and again by science. In this sense Parmenides can be seen as clearing the way for such later advances as Einstein's theory of relativity, Boltzman's laws of thermodynamics, the atomic theory, and a whole host of philosophical schools ranging all the way from George Berkeley's idealism to G.W. Leibniz's rationalism.
In the wake of Parmenides' radical claims, two new schools of Presocratic philosophy sprung up, one with the aim of extending Parmenides' conclusions and another with the aim of limiting them. The first of these schools, commonly referred to as the "Eleatics", included such famous thinkers as Zeno (with his riddling paradoxes) and Milesses. The latter school, the Pluralists, included the Atomists, Empedocles, and Anaxagoras.
Though the age of the Presocratics officially ended with the arrival of Socrates on the philosophical scene, in roughly 450 B.C., the real end to their era of thought came with the arrival of Aristotle in 384 B.C. Socrates, unlike the Presocratics, was more interested in questions about human life than in questions about the natural world, and while his student Plato showed some interest in the more scientific questions, Plato's primary interest lay in the human sphere as well. Aristotle, on the other hand, was a direct intellectual descendant of the phusikoi. His interest was in the natural world, and his inquiries were heavily guided by the work already done by the Presocratics. Though he was highly critical of his predecessors, he was also greatly in debt to them. His notion of the four elements, for instance, was taken directly from Empedocles, and his entire physics is, in essence, a reaction to Parmenides. Still, his theories took philosophy in radical new directions, making the Presocratic way of thought a thing of the past.