How the Presocratic Philosophers Invented Critical Rationalism

Thales of Miletus, a Presocratic philosopher born in 626 BCE, proposed that the underlying, fundamental substance of all matter was water, while his student Anaximander thought the substance was an indefinite material called Apeiron. Anaximenes, Anaximander’s student, disagreed with both and thought the fundamental substance was air, while Heraclitus disagreed with everyone and thought the substance was fire. 

On the surface, this does not seem to be a very impressive list of accomplishments in light of what we now know about physics and cosmology. What is impressive, however, is that each philosopher had their own elaborate naturalistic explanations in an age dominated by supernatural beliefs (as has been pointed out many times). 

But there is another, even more profound lesson hidden in the modern irrelevancy of their ontological theories, which was first explained by Karl Popper in his essay titled The Beginnings of Rationalism. Popper noticed the surprising fact that all of the above philosophers disagreed with each other despite coming from the same school

Disagreements coming from different schools of thought are obvious and expected. We expect, for example, the Sophists to disagree with the Platonists and the Christians to disagree with the Muslims. The natural course of affairs is for people to identify with a group that defines itself in contrast to other groups, and then to defend the integrity of their chosen group against the doctrines of outsiders. This is the well-known phenomenon of tribalism that plagues the human race and the modern world. 

What is exceptional about the Ionian school of philosophy is that the students actively and vigorously disagreed with their teachers and with the founder of the school, and, what’s more, that this must have been encouraged by the school’s founder, Thales. As Popper wrote:

“If we look for the first signs of this new critical attitude, this new freedom of thought, we are led 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. But there is no trace in the sources of a story of dissent, of any quarrel, or of any schism.

“This suggests, I think, that it was Thales who founded the new tradition of freedom—based upon a new relation between master and pupil—and who thus created a new type of school, utterly different from the Pythagorean school. He seems to have been able to tolerate criticism. And what is more, he seems to have created the tradition that one ought to tolerate criticism…Yet I like to think that 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.”

If Popper is right, which I think he is, this is perhaps the first major turning point in the history of philosophy. Dogmatic schools of thought, including most religions, survive via the dissemination of orthodoxy and an unwavering reverence for the founders. If you go against orthodoxy in such a system, you risk ostracization or worse. 

Thales and the Presocratics established a new system of active and encouraged disagreement that allowed for the possibility of progress in knowledge, based on a recognition of human fallibility and our inherent cognitive limitations. Xenophanes captures the idea well:

“The gods did not reveal, from the beginning,
All things to us, but in the course of time
Through seeking we may learn and know things better.
But as for certain truth, no man has known it,
Nor shall he know it, neither of the gods
Nor yet of all the things of which I speak.
For even if by chance he were to utter
The final truth, he would himself not know it:
For all is but a woven web of guesses.”

This is an expression of the critical rationalism that Karl Popper would advance thousands of years later. Critical rationalism is founded on the concept that no knowledge is final, and that the more we learn, the more problems and questions arise. The best we can do is to say, “this is my theory, my best logical and coherent explanation of how the world works based on the evidence. But it is not final; if you disagree with it, do your best to criticize my theory and replace it with something better.” This spirit of intellectual humility and willingness to challenge orthodoxy is the driving force behind all growth in knowledge, and we owe its original development to the Presocratics—their top collective contribution to civilization.

This is in fact the underlying mechanism that western philosophy will emulate for the next 2,500+ years. Plato revered Socrates but expanded on his philosophy, for instance, just as Aristotle, who studied under Plato at the Academy for 20 years, turned Plato and his theory of forms on its head. All subsequent philosophers of note in the history of the subject would either expand upon or reject the teachings of their mentors—not simply evangelize for them. Even in science, we see progress only when ideas are challenged, as when Albert Einstein improved upon the models of Isaac Newton by rejecting Newton’s ideas of absolute space and time. 


What can we learn from the Presocratics today? The lesson is clear: that we should all be more skeptical of the people we revere and critical of the traditions in which we are raised. Our ancestors were wrong about many of their beliefs and we are no doubt wrong about many of ours. History tells us that knowledge is provisional and grows only through sustained criticism; not, as is sometimes believed, by searching for certain truth as revealed to a select few individuals in our distant past. 

Further Reading

The Presocratic Philosophers

The Presocratic Philosophers: A Critical History with a Selection of Texts by G.S. Kirk

Presocratic Philosophy: A Very Short Introduction by Catherine Osborne.

Internet Encyclopedia of Philosophy: Presocratics

Karl Popper and Critical Rationalism

For a concise overview of Karl Popper’s ideas on critical rationalism, check out Philosophy and the Real World: An Introduction to Karl Popper by Bryan Magee. 


For a selection of Karl Popper’s best non-technical writings, including the essay The Beginnings of Rationalism, read Popper Selections, and for two modern interpretations of Popper’s ideas, read Critical Rationalism: A Restatement and Defence by David Miller and The Beginning of Infinity: Explanations That Transform the World by David Deutsch.

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