Ancient Greek Philosophy and the Birth of Neuroscience

We recognize today that the Ancient Greeks made significant achievements in mathematics, engineering, and astronomy, and that their achievements in these fields went unrivaled (in Europe) until the Renaissance. We know, for example, that Aristarchus proposed a heliocentric model of the solar system and offered a reasonably accurate estimation of the size of the Earth, and did so in the third century BCE – a full eighteen centuries before Copernicus. We also know that the ancient Greeks developed both geometry and the concept of a formal proof, and contributed to later ideas in mathematical analysis, number theory, and applied mathematics. Some think they might have been close to developing integral calculus. And they were skillful engineers, building such devices as alarm clocks, vending machines, automatic doors, and even the first analog computer.

These, at least, are the achievements of which I was aware. It recently occurred to me that I knew nothing about the Greeks’ knowledge of the brain. I knew that Aristotle thought that the mind was in the heart, and that the brain’s main purpose was to cool the blood. His theory seemed obviously wrong so I never thought to dig deeper and see whether other ancient Greek thinkers had produced any ideas that come closer to the discoveries of modern neuroscience. I was pleasantly surprised by what I found.

I came by this review of the contributions of ancient Greek philosophy to neuroscience, written by medical researchers Enrico Crivellato and Domenico Ribatti of the University of Udine in Italy. The paper details the neuroscientific and psychological theories of Greek philosophers, physicians, and anatomists, which I highly recommend reading if you have the time. The review is quite extensive, so I thought that here, I’d focus on some of the main concepts of the paper as well as discuss some of the really impressive hypotheses that came out of ancient Greece.

Of particular note is the division of ancient Greek thinkers into two camps, encephalocentrism and cardiocentrism. Encephalocentrism is the theory that the mind is in the brain, and cardiocentrism holds that the mind is in the heart. At the time that these two schools of thought emerged, the very notion that the “mind” was a singular entity that could be localized to any single organ was new. Before encephalocentrism and cardiocentrism came to dominate the intellectual discourse surrounding the mind around the 5th century BCE, philosophers in Homeric times (around the 8^th century BCE) thought that there were multiple “souls,” each associated with a different aspect of mentality and each localizable to a different body part.

Probably the first encephalocentrist thinker was Alcmaeon of Croton, around the early 5^th century BCE. Alcmaeon was the first physician to dissect animal corpses. He held that all sensation is conveyed to the brain through “poroi,” which are channel-like structures. He described two poroi connecting the eyes to the brain, which were probably the optic nerves. As an empiricist, he observed that if the brain is moved, sensation is compromised, and so he correctly concluded that the brain is the seat of consciousness. A number of other philosophers of his time made similar observations, but none were more impressive than the famed physician Hippocrates of Cos. Hippocrates localized both the intellect and neurological diseases to the brain. In his treatise De morbo sacro, he argued that epilepsy is due to a problem in the brain, and is not, as had been previously thought, a “sacred disease.” He writes, “Its [epilepsy’s] supposed divine origin is due to men’s inexperience and to their wonder at its peculiar character.” In the same treatise, he claims that the brain gives rise to emotions, aesthetics, and judgment. What is most remarkable is his observation that a cut into one side of the brain produces a bodily spasm on the opposite side of the body, and that loss of speech (which we now know is due to damage in areas that are almost always in the left hemisphere) is associated with paralysis of the right side of the body. Further research supporting the lateralization of brain function was not conducted until the 19^th century.

We now think that the opposing view – cardiocentrism – is obviously wrong. But thinkers as prominent as Aristotle subscribed to this view. Why? One possible explanation is that these philosophers observed that when the heart stops beating, you die, and so they conjectured that the heart must control the mind. Many prominent ancient Greek physicians held this view, and indeed the debate between cardiocentrism and encephalocentrism continued well into the Renaissance.

Two other Greek physicians worth mentioning are Herophilus of Chalcedon and Erasistratus of Ceos. Both discovered and described a number of structures in the brain, identified cranial and spinal nerves, and, perhaps most impressively, differentiated motor and sensory nerves. Erasistratus made the particularly remarkable observation that human intelligence might have something to do with the convolutions of the human neocortex. We now know of course that cortical convolution or “gyrification” provides greater surface area relative to brain volume and hence greater computational capacity. Galen, the famed Roman physician, rejected Erasistratus’ idea. Crivellato and Ribatti write, “Had Erasistratus’ conjecture been further properly investigated, the history of neuroscience could have run a different way.”

Unfortunately, most of what the Greeks knew about the brain was forgotten for millennia. The destruction of the Library of Alexandria erased almost all records of ancient Greek thought, and scientific research in Europe (though not in the Muslim world) stagnated for centuries. But what if Greek science never slowed down? What would the world look like today? Carl Sagan writes the following in his Cosmos:

What if the scientific tradition of the ancient Ionian Greeks had survived and flourished? That would have required many of the social forces of the time to have been different – including the prevailing belief that slavery was natural and right. But what if that light that dawned in the eastern Mediterranean 2,500 years ago had not flickered out? What if science and the experimental method and the dignity of crafts and mechanical arts had been vigorously pursued 2,000 years before the Industrial Revolution? What if the power of this new mode of thought had been more generally appreciated? I sometimes think we might then have saved ten or twenty centuries. Perhaps the contributions of Leonardo would have been a thousand years ago and those of Albert Einstein five hundred years ago.

…

If the Ionian spirit had won, I think we – a different “we,” of course – might now be venturing into the stars. Our first survey ships to Alpha Centauri and Barnard’s Star, Sirius and Tau Ceti would have returned long ago. Great fleets of interstellar transports would be under construction in Earth orbit – unmanned survey ships, liners for immigrants, immense trading ships to plow seas of space. On all these ships there would be symbols and writing. If we looked closely, we might see that the language was Greek. And perhaps the symbol on the bow of one of the first starships would be a dodecahedron, with the inscription  “Starship Theodorus of the Planet Earth.”

Carl Sagan was, of course, an astronomer. And his lament at the millennia-long hiatus in the spirit of rational inquiry paints for us a picture of what cosmology and space exploration could have been like by today. But what of neuroscience? Or cognitive science more generally? We might have cured all neurological diseases by the 15^th century. Or we might now be developing the “psychohistory” of Asimov’s Foundation series, which would allow us to accurately predict the behavior of large groups of people. Or we may have long ago pushed past our biological limitations with brain-computer interfaces and become post-human. Who knows? I just hope that our current rate of scientific discovery continues unabated, so that we will, in due time, achieve what we could have achieved centuries ago.