Imagine living in a time when the fear of gods colored the perception of nearly every event. Picture the sound of thunder as the violent crash of Zeus’s lightning bolts hurled down from the heavens, or the blinding arc of the Sun as the god Helios racing his chariot across the sky. Picture, too, the tremors of earthquakes—destroying cities and spawning tsunamis—as the furious strike of Poseidon’s trident.
This was the world of Ancient Greece—the world before science—where everyday events were often filled with awe—and terror. It was a world dominated by gods, omens, oracles, and the vicissitudes of a more precarious, anxiety-ridden life.
Against this backdrop, the philosophers of the Ionian school—Thales of Miletus (c. 620 — c. 546 BCE) chief among them—were the first to cast aside this grim and frightening mythology in the search for materialistic causes of natural events—and to therefore begin to replace fear and volatility with the possibility of rational prediction and control. And although they weren’t strictly atheists, they took the initial step in the long historical process of banishing the gods from the operations of the natural world.
The Ionian philosophers each had their own theory for the primary substance underlying all matter. Thales—Western history’s first recorded philosopher—thought that everything was made of water; Anaximenes, belonging to the same school, thought it was air; and Heraclitus, selecting among the remaining elements, thought it was fire. Anaximander, probably the most sophisticated of the bunch, thought it was some boundless, eternal substance called apeiron.[1] And all of them, crucially, refused to attribute to natural phenomena the actions of temperamental gods.
But the Ionians, and Thales in particular, gave us something much grander than even this. Obscured by the primitive nature of their physical speculations, the Ionians unassumingly gave us the intellectual tools that would drive all subsequent progress in knowledge—methods of thinking that would ultimately lead to the establishment of modern science, and with it, the rejection of supernaturalism in all its inglorious forms.
The history of atheism, then, must begin with Thales.
Thales’ Invention of Critical Rationalism
It takes a reflective mind to propose that, despite all the apparent variety in the world, everything, at bottom, is made of some unitary, fundamental substance. As far as we know, Thales was the first to do this, and introduced the idea that there is a difference between appearance and reality—that our senses can deceive us—and that the investigation of the world should be predicated on the idea that reality is often hidden from our commonsense notions of how things seem. Thales was also the first to suggest that this investigation can (and should) be done materialistically, not by invoking mythological figures and creatures as stand-ins for our ignorance.
And so that’s what Thales of Miletus—one of the Seven Sages of ancient Greece—set out to do. In addition to proposing that all is made of water, Thales also believed that the earth rests, or floats, on water. If the earth, as it was believed, was stationary and central to the universe, then an explanation was needed as to what supported the earth in this preeminent position. Thales, fittingly, thought the earth floated on a cosmic ocean like a piece of wood floats in the sea.[2]
What happened next is nothing short of extraordinary—the greatest gift the presocratic philosophers gave us. It will take a bit of explaining to convey its importance.
Anaximander, Thales’ pupil, and part of the same Ionian school, proposed a radically different hypothesis concerning the position of the earth. The earth, according to Anaximander, floats freely in the center of the universe.
According to the philosopher of science Karl Popper, Anaximander likely arrived at this conclusion by criticizing Thales’ assertion that the earth rests on water. [3] Anaximander noted that, if the earth is supported by water, we would then need to know what supported the water itself, and then what supported that, ad infinitum. Anaximander therefore hit upon the problem of infinite regress (the same problem creationists encounter when asked who created God). To prevent this regress, Anaximander proposed that the fundamental substance of the universe is eternal and boundless and that, if the earth is at the center of this space, it has nowhere to go, nowhere to fall, nowhere to be pushed.
This is quite remarkable in itself, and would ultimately lead to further cosmological theories that allowed for the movement and position of astronomical bodies around the circumference of the earth. But what it more importantly signifies—from an epistemological perspective—is that the path towards progress in our understanding of nature lies in the free criticism of ideas—even, and especially, among adherents of the same school.
It’s disheartening to realize just how uncommon this was throughout much of human history, and even among the Greeks. The Pythagoreans, for example, strictly forbade the eating of beans, because Pythagoras, the school’s founder, linked them with reincarnated souls. If you were to argue against this doctrine—and help yourself to a hearty serving of baked beans in defiance—you could kiss your days as a Pythagorean goodbye.
Even the later schools of Plato and Aristotle (albeit to a lesser extent), mostly considered the founder of their respective traditions to be virtually infallible and beyond question. This is why, despite the achievements of Aristotle himself, the Aristotelian tradition held back the progress of science for so long.
Aristotle ultimately championed a deductive, authority-based, and teleological framework that dominated Western thought for nearly 2,000 years. Aristotle’s list of errors includes a geocentric (earth-centered) view of the universe, incorrect theories of motion, and spontaneous generation in biology, all based on—and this is the crucial part—untested assumptions, without the aid or encouragement of experimentation. When ideas are shielded from criticism in this way, under the cloak of authority, progress simply cannot be made.
The sins of the monotheistic religions, in terms of institutional rigidity, were far more egregious. Intellectual inertia is literally built into scripture; if you’re a Christian, for instance, you had better not criticize, reject, or otherwise alter any of the teachings of Jesus (or whatever happens to be considered orthodoxy in your community), else you cease to be a Christian. Go against this orthodoxy, and you face the charges of heresy or blasphemy—punishable by ostracism or, in some historical cases, death. In other words, criticism is not tolerated; the study and promulgation of established, infallible, permanent knowledge is all that is required—and expected—in closed systems of thought. Dogma is the name of the game.
The dogmatic transmission of knowledge even applied, albeit to a lesser extent, to Plato’s Academy. Plato would later complain that Aristotle—who had studied under Plato at the Academy for 20 years—in criticizing his theory of Forms, “spurns me, as colts kick out at the mother who bore them.”[4] You simply were not expected to challenge your masters. And if you did, they did not hide their displeasure. One may even speculate that this, in part, is why Aristotle was not named Plato’s successor.
But look what happened with Anaximander. Not only did he criticize Thales, he also flat-out rejected him. He proposed an entirely different theory on the grounds that, due to the problem of infinite regress, Thales’ account wasn’t even coherent. And yet, there seems to be no indication that Thales was ever upset about any of this. Anaximander was not labeled a heretic and expelled from the school, forced to establish his own. He remained a part of the Ionian tradition, doing what Thales must have encouraged—thinking critically about the nature of the world, wherever that led; subsequent thinkers of the Ionian school followed due course. As Popper wrote:
If we look for the first signs of this new critical attitude, this new freedom of thought, we are led back to Anaximander’s criticism of Thales. Here is a most striking fact: Anaximander criticizes his master and kinsman, one of the Seven Sages, the founder of the Ionian school. He was, according to tradition, only about fourteen years younger than Thales, and he must have developed his criticism and his new ideas while his master was alive. (They seem to have died within a few years of each other.) But there is no trace in the sources of a story of dissent, of any quarrel, or of any schism.
Thales, it seems, actively tolerated criticism among his students, yet Popper thought he did even more than this: “I can hardly imagine a relationship between master and pupil in which the master merely tolerates criticism without actively encouraging it.”
This is highly plausible; students trained in a dogmatic school would hardly be so keen to openly contradict their master in such a thoroughly destructive way. It would be like a Catholic theologian advancing the idea that Jesus had no divine relationship to God whatsoever, but was simply a wise human teacher, and the Church being totally fine with it.[5] That this idea seems so laughably improbable to us proves the point, and convinces us that Thales probably did actively encourage the critical spirit in a school where every thinker proposed an entirely different theory.
Whatever the case may be, it seems that, for the first time in Western history, a tradition emerged in which foundational claims about nature were treated as conjectural and open to internal criticism, rather than as authoritative truths. The days of Zeus, Poseidon, and Helios would soon be numbered.
Paving the Way for Scientific Progress
According to Popper, there is only one true theory of knowledge, “the theory that knowledge proceeds by way of conjectures and refutations.” Popper advanced the notion that, when asked how you can personally know something—even as a scientist—the proper response is, “I don’t; I only propose a guess. If you are interested in my problem, I shall be most happy if you criticize my guess, and if you offer counter-proposals, I in turn will try to criticize them.” Under this epistemological framework, all knowledge is rendered provisional and subject to revision through open dialogue and critical debate (facilitated by new experimental evidence). It’s a wonderful system, and a big part of the reason that we no longer believe earthquakes are caused by Poseidon’s trident.
The methodology of the Ionian school—where every new generation of thinkers actively challenged and superseded the theories of their predecessors— perfectly anticipates, even if not explicitly articulated, Popper’s epistemology, along with the emergence of modern science, humanity’s greatest invention. What is remarkable is that this happened over 2,600 years ago, at the very birth of natural philosophy, by Thales—the initiator of this noble tradition.[6]
Thales, in fact, will serve us well in our journey through the history of doubt. Acting as our guide and mentor, Thales will remind us to remain open and curious, to question even our most cherished beliefs, and to commit allegiance exclusively to the truth, even if that means boldly overturning protected dogma.
It was left to other thinkers, however, to apply this new critical attitude to orthodox religion. For that, we must turn to Western history’s earliest recorded religious skeptic, Xenophanes of Colophon.
Notes
[1] G. S. Kirk, J. E. Raven, and Malcolm Schofield, The Presocratic Philosophers: A Critical History with a Selection of Texts, 2nd ed. (Cambridge: Cambridge University Press, 1983).
[2] Patricia O’Grady, “Thales of Miletus,” Internet Encyclopedia of Philosophy, last accessed February 8, 2026, https://iep.utm.edu/thales/.
[3] Karl R. Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (London: Routledge, 2002).
[4] Diogenes Laërtius, The Lives of Eminent Philosophers, trans. R. D. Hicks, Loeb Classical Library (Cambridge, MA: Harvard University Press; London: William Heinemann, 1925), Book 5, chap. 1, 181.
[5] Incidentally, we know how the Church would react to this. Giordano Bruno made exactly these claims—that Jesus was not divine—and was subsequently burned at the stake. Not exactly the same approach as Thales.
[6] Even if Thales did not explicitly articulate anything like a modern epistemology, it was implicitly carried out, and therefore the lesson holds.
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