It doesn’t appear that anybody has noticed yet, but 2021 marks a rather important anniversary in the history of science and western civilization. It was 2,500 years ago this year that a philosopher named Anaxagoras arrived in Athens, Greece.

Nobody held any celebrations at the time, either. But it was nonetheless an important historical and intellectual landmark. Before Anaxagoras, ancient Greek science (or to be less anachronistic, natural philosophy) hadn’t actually been practiced much in Greece itself. Natural philosophy originated early in the sixth century B.C. at the Greek settlement Miletus in Ionia, the western coast of modern-day Turkey. A second branch of primordial Greek science soon took root in southern Italy after one Ionian, a math fan named Pythagoras, moved there.

Anaxagoras, born in the Ionian town of Clazomenae, was the first natural philosopher to reside in Athens and promote the Ionian philosophical outlook there. As the science historian George Sarton wrote, Anaxagoras “introduced the scientific spirit into Athens.” Soon after, Athens became the western world’s center of philosophical inquiry, as the triumvirate of Socrates, Plato and then Aristotle established philosophy as an essential component of civilized intellectual discourse.

To be honest, there is some doubt about the exact date of Anaxagoras’ move to Athens. But the biographer of philosophers Diogenes Laertius wrote that Anaxagoras began to do philosophy in Athens at the age of 20, and says he was 20 years old when the Persian king Xerxes attacked Greece and that was 480 B.C., 2,500 years ago. (You might think that 2021 would make that 2,501 years ago, but only if you forgot that there was no year 0, so you have to subtract one year from the calculation.)

It’s possible that during his time in Athens, Anaxagoras met the young Socrates, but the direct link to Socrates and his philosophical descendants was through the philosopher Archelaus. Anaxagoras “was the first to stimulate Archelaus the Athenian to practice philosophy,” wrote the famous physician Galen. And Archelaus was the teacher of Socrates, who taught Plato, who taught Aristotle, whose influence dominated science for two millennia.

Anaxagoras came along about a century after the first Ionian philosophers, Thales of Miletus and his younger Milesian contemporary Anaximander. Along with a third Milesian, Anaximenes, they had established a new way of viewing the natural world. They sought explanations for phenomena in natural causes, rather than ascribing them to the behavior of mythological gods invented by poets to explain cosmic history. No longer was lightning a sign of an angry Zeus, declared Anaximander — rather it flashed when clouds were disrupted by the wind.

While the original Milesians did not agree on all issues, they did all insist that natural philosophy should be based on an underlying foundation (called the arche or arkhé), a principle from which all of reality could be derived. “The notions of beginning, origin, governing principle and cause were closely united in the single word arche,” wrote the philosopher-historian William Guthrie.

Thales thought that the fundamental principle was water; Anaximenes said air. Anaximander thought everything arose from a mysterious material called the apeiron, which means something like the unlimited or the boundless. Over in Italy, the Pythagoreans promoted the idea that the underlying foundation of everything was number.

For Anaxagoras, the arche was nous, or “mind” (sometimes translated as “intellect” or “intelligence”). His approach extended the scientific ideas of his Ionian predecessors to address issues posed by the Italian philosopher Parmenides. Everything had to always be as it is, Parmenides reasoned, because nothing could come to be out of nothing — nonexistence could not produce existence, because there is no such thing as nonexistence, by definition of existence. Reality consisted in an ever-present, unchanging, unmovable mass of undifferentiated sameness that filled all of space, Parmenides concluded. There was therefore no room for any motion, or any change — the world perceived by the senses was false, an illusion concealing the true nature of reality. Senses offered a “way of seeming”; only reason provided the “way of truth.”

Although it sounds whacko to modern ears, in those days it was a hard argument to refute. But Anaxagoras had a sophisticated and subtle mind; in responding to Parmenides, he introduced an entirely new idea about fundamental reality, contending that all the different sorts of matter are already present in any given piece of matter. No new thing has to “come into being,” because all possible things already exist to begin with and continue to exist in everything — even if in amounts too small for the senses to detect. A lump of supposedly pure gold, for instance, also contains tiny “seeds” of every other sort of matter. Our senses are just too coarse to notice the seeds. (For that matter, the seeds themselves contained smaller amounts of everything, also. Anaxagoras had conceived the notion of infinite divisibility, another novel thought.)

Any piece of matter could seem to change into something else by virtue of shifts in the relative amounts of its seeds. Eating vegetables, for instance, could produce flesh and bone in your body because the digestive process concentrated the flesh and bone seeds that were imperceptibly diffuse in the original food.

Initially, all matter was one big static mass. At some point in the past, nous, or mind, set that mass into whirling motion, concentrating heavy stuff (like earth) in the middle, creating the Earth. Chunks of earth whirled outward became stars and the sun and the moon.

Anaxagoras’ nous was the one distinctive ingredient in his system. Other stuff was all mixed with everything else. But mind was its own thing. “Mind is something infinite and independent and is mixed with nothing,” he wrote. But mind (while maintaining its purity) is present in many things, including all people, called by Anaxagoras “the wisest of animals.” (Guthrie recounts that when asked why some people don’t seem so wise, “Anaxagoras is said to have remarked that though all men have intellect they do not always use it.”)

Despite his own considerable intellect, Anaxagoras’ theory of matter was wrong. But his reputation rests on many other contributions to scientific thought. A century ago, Thomas Heath, the eminent scholar of Greek science and math, declared that Anaxagoras was “a great man of science” who “enriched astronomy by one epoch-making discovery”: that the light of the moon is not its own, but a reflection of light from the sun. (Some scholars say he got the idea from Parmenides, but in any case, it is still very deserving for a crater near the moon’s north pole to be named Anaxagoras.) 

Anaxagoras wrote a treatise covering many other scientific subjects, including meteorology and geology. He supposedly predicted that a rock could fall to Earth from the sky; in any case, he was awarded credit for such a prediction when a meteorite fell in Thrace (now Turkey) in 467 B.C. He argued that the Earth was flat, though. And that it was supported in space by the air beneath it, echoing his Ionian predecessor Anaximenes.

Anaxagoras’ scientific importance rests, however, not on the accuracy of his theories but rather on the insightfulness of his attitude. He expressed a scientific attitude renouncing the supernatural more clearly than his predecessors. Even Thales felt that “there were gods in all things,” Aristotle had written, and Thales and others had attributed souls to heavenly bodies. From his known writings there is no sign that Anaxagoras’ nous was in any sense religious — it was a natural component of the cosmos, giving it direction, just as the human mind induces a human body to move its limbs. “He nowhere in the extant material identifies mind with a divine principle or god,” as one scholar has noted.

Even more profoundly, Anaxagoras identified a key issue that has perplexed practitioners of science ever since — the relationship between reason and the senses. It was absolute devotion to reason — and absolute disregard for the senses — that led Parmenides to declare reason the way of truth and sensible phenomena illusory.

Anaxagoras fully agreed that the senses could be misleading, calling them “feeble” and unable “to distinguish what is true.” Human senses are simply not acute enough to perceive reality in full HD clarity. There is more to reality than what we can see. But — the key point — with senses complemented by intellect, we can infer much about the deeper, unseen reality from what we do see, Anaxagoras realized. “Appearances are vision of things that are invisible,” he wrote, or in another translation, “phenomena are a sight of the unseen.” Reality is richer than what it immediately appears, yet the human mind remains capable of exploring it, and finding out a lot about it. And with that realization, progress in the scientific understanding of reality became possible.

Naturally enough, Anaxagoras’ emphasis on natural explanations and his disdain for the gods got him in trouble. Athenian officials charged him with impiety, convicted him and sentenced him, by some accounts to death, by others just to prison. His friend the Athenian politician Pericles intervened to arrange banishment, and Anaxagoras spent his last years in Lampsacus, a city in what is now northwestern Turkey, where he was revered as a champion of mind, and truth.