John Philoponus was one of the last great original thinkers of Greek antiquity, and he was the first person to begin a significant, wide-ranging assault on Aristotle’s flawed physics.
Philoponus originated one of physics’ great paradigm shifts: he said that after a projectile is launched, it keeps moving because of a force impressed into it by the thrower. Until then, following the lead of Plato and Aristotle from eight centuries earlier, people said the force was transferred to the air, not the projectile. Galileo cited Philoponus’ work when formulating his new physics.
Philoponus shattered another paradigm when he said the planets were not divine. He said they moved for the same reason that objects on the earth moved – i.e. that a force of motion had been impressed upon them.
Over a thousand years before Galileo, Philoponus said that contrary to Aristotle’s physics, two different weights dropped from the same height did not fall at a rate proportional to their weights.
John Philoponus was born near the end of the fifth century, most probably around 490 AD. No details of his birthplace or his parentage survive. We do not know what he looked like.
Like most educated people in the Eastern Mediterranean in late antiquity, John spoke Greek.
The name ‘John’ implies that he was born into a Christian family.
John lived and worked in the Greco-Roman city of Alexandria, Egypt, hence he is sometimes called John of Alexandria.
As the author of two Greek grammar books, he is sometimes called John the Grammarian.
The name Philoponus has two possible sources: in Greek it means ‘lover of work’ and this seems appropriate, because he was a prolific author. On the other hand, he could have been a member of the Philoponeion, an order of Christian lay workers.
Philoponus completed his formal education at Alexandria’s Neoplatonist school run by Ammonius, a former student of Proclus in Athens. In addition to philosophy, the school placed strong emphasis on mathematics and astronomy.
The Christian authorities of Alexandria held the Neoplatonist school in high regard. They were happy for it to operate in a scholarly way provided it did not push beliefs that contradicted Christian dogma. For example, Christians said God had created everything during the Creation. Neoplatonists disagreed, saying that matter had always existed, courtesy of God.
Neoplatonism and Christianity shared some tenets, such as belief in a single transcendent God. During at least part of Philoponus’ lifetime it was possible, in the style of Rome’s great philosopher and senator Boethius (c. 477 – 524 AD), to be both a Christian and a Neoplatonist.
Lifetimes of Selected Greek Scientists and Philosophers after the Golden Age
John Philoponus lived in an era separated by many centuries from Greece’s greatest philosophers, mathematicians, and scientists: Archimedes died 700 years before Philoponus was born.
Foreshadowing Newton’s First Law
The Law of Inertia
A question that had tormented the minds of early philosophers was why a stone should continue to fly through the air after it left the hand of the thrower.
Today we explain this by citing Isaac Newton’s First Law of Motion, also known as the Law of Inertia, which says of a moving body:
- A body will continue to move uniformly in a straight line unless acted upon by an external force.
After a stone leaves a thrower’s hand, Newton said that the stone deviates from moving in a straight line because gravity imposes an external vertical force. The overall path of the stone is a parabolic curve. Wind resistance is another external force acting on the stone, causing it to lose speed in the horizontal direction. All of this was noted by Galileo Galilei decades before Newton wrote his laws.
Aristotle is Right and Wrong
In Book 4 of Physics written 800 years before John Philoponus was born, and 2,000 years before Newton wrote his laws of motion, the great philosopher Aristotle stated the principle of inertia more or less correctly:
“No one could say why something that is moving should stop anywhere; why should it stop here rather than there? Therefore a thing will either be at rest or must move forever, unless something more powerful get in its way.”
The trouble is, Aristotle decided this view was false, because its falseness helped bolster his flawed assertion that a vacuum was impossible. He believed air was needed to keep a thrown stone moving until the stone’s natural movement (falling to the ground) took over.
Aristotle believed that if he threw a stone through the air, the force he applied moved the stone only while he was touching it. After he released it the stone kept moving because the air displaced by the stone’s forward movement rushed around behind the stone and continued to push it forward. A similar argument for projectiles was put by Aristotle’s teacher Plato in his work Timaeus.
Aristotle said that in addition to transmitting the force of movement, air also hindered it. The stone’s natural movement would eventually prevail and the stone would drop to the ground.
Philoponus Breaks New Ground
Philoponus was one of the few original thinkers of his age. Rather than accepting the truth of Aristotle’s argument, he challenged it. Over a thousand years later, in the early 1600s, Philoponus’ challenge would become one of the foundations of Galileo’s demolition of Aristotle’s physics.
Philoponus began undermining Aristotle in a commentary he wrote on Aristotle’s Physics in about 517 AD, mocking the idea of air rushing around behind a thrown stone to keep it moving. Philoponus said if Aristotle were correct about this, then soldiers need not launch projectiles. They could perch them on a thin wall and set the air in motion behind them with bellows and watch them fly off. Obviously, this does not work and Philoponus said that although the projectiles might move, they would move just far enough to drop off the wall.
Philoponus said that the stone kept moving after it left the thrower’s hand because the hand had implanted movement into the stone by the action of throwing it. Philoponus’ concept of an implanted force is similar to the later concept of inertia found in Galileo’s work and in Newton’s First Law.
With Philoponus the motive force is implanted in the stone by the thrower. Before Philoponus, the belief was that the motive force was implanted in the air by the thrower.
However, unlike Aristotle, Philoponus does not seem to recognize that the stone loses energy because of air resistance. He suggests simply that the implanted force in the stone becomes exhausted. Philoponus’ commentary on Book 8 of Aristotle’s Physics, where he discusses this issue most fully and no doubt clarifies his own thoughts, unfortunately no longer exists.
One Law for the Whole Universe
In his work De opificio mundi – The Creation of the World, which began circulating sometime around 546 – 560 AD, Philoponus extended the implanted impetus argument to the heavenly bodies. As a Christian, Philoponus said God gave the heavenly bodies impetus when he created them. Although this idea may seem unremarkable today, in those days it was revolutionary.
Philoponus had made an incredibly important connection in physics. He said the movements of planets and other heavenly bodies were governed by an implanted motive force, just as movements of projectiles on the earth were. This was a major break with Aristotle’s traditional belief that matter in the heavens was divine matter, which behaved differently to matter on the earth because different rules applied.
Philoponus’ ideas bring to mind Newton’s later inspiration that the force causing an apple to drop to the ground was the same force that kept heavenly bodies in orbit – the fundamental idea is that same physical laws apply to the heavens and the earth.
However, having made a brilliant conceptual breakthrough in physics, Philoponus spoils it somewhat for modern scientists by adding that God also implants the souls of animals with their movements as he does the upward movement of fire.
Dropping Weights – Falling Bodies
Aristotle believed heavier objects fall faster than lighter objects. He said that the time taken for a weight to fall was inversely proportional to its weight, hence:
Galileo gathered further evidence for the relationship between weight and rate of fall by using a water-clock to time balls rolling down wooden slopes.
Over a thousand years before Galileo’s time, Philoponus foreshadowed his work.
Philoponus reviewed Aristotle’s claims, comparing them with an experiment dropping different sized weights.
He said that, contrary to Aristotle’s ideas, heavier weights did not fall much faster than lighter ones. The inverse proportionality claimed by Aristotle did not exist:
However, Philoponus did not make the full conceptual breakthrough that Galileo did. Philoponus continued to believe that heavier weights should fall faster than light ones, even if the difference in their rates of fall was much smaller than the one claimed by Aristotle’s theory.
The Significance of Philoponus
Over a thousand years after Philoponus wrote about projectiles and falling weights, his words guided the thoughts of Galileo Galilei as he pondered the problems of motion. Science historian William A. Wallace notes that in Galileo’s early writing, he cites Philoponus more frequently than better-known authorities such as Plato, Albertus Magnus, and John Duns Scotus.
In his book Prelude to Galileo Wallace writes that:
- For Aristotle, nature was the cause of motion and rest.
- For Philoponus, force was the cause of motion and rest.
Philosopher and science historian Michael Wolff writes in Philoponus and the Rejection of Aristotelian Science:
“…it was Galileo whose early writings led historians of science to notice impetus theory as a rudimentary form of classical dynamics and to notice John Philoponus as a predecessor of impetus theory.”
Wolff notes that the great polymath Alexander von Humboldt wrote in his second volume of Kosmos that Philoponus produced the only work in antiquity that facilitated physical astronomy’s evolution into celestial mechanics.
In The Structure of Scientific Revolutions Thomas Kuhn described impetus theory as a late medieval paradigm shift, without crediting the original concept to Philoponus, who originated it in the early medieval period.
Is a Vacuum Possible?
Contrary to Aristotle, Philoponus wrote that the existence of a vacuum [void] is possible. In this he supported Democritus, who was alive at the same time as Aristotle in the fourth century BC. Democritus argued that everything consisted of indivisible atoms and void.
Aristotle said a vacuum was impossible because in a vacuum there would be no resistance to motion, so the speed of something falling in a vacuum would be infinite. He argued that since infinite speeds were absurd, then so was the idea of a vacuum.
Philoponus countered with the argument that with no resistance to motion the extra time required to overcome resistance would be saved, so a movement would take less time in a vacuum. It would still take some time, so speed in a vacuum would not be infinite.
Light and Heat
Aristotle’s position was the sun was not inherently hot, but heated the earth by friction. Philoponus agreed with Plato that the sun was a place of fire.
Philoponus agreed with Aristotle that light was not material – it had no substance or physical form. He believed light had direction, but wrongly believed it did not travel. After light was emitted, he believed it arrived instantly everywhere at the same time.
In this, he differed from Empedocles, who flourished a century before Aristotle.
Of course, these were purely philosophical arguments with no experimental support. A thousand years later, when Galileo tried to measure the speed of light by experiment, he concluded that its speed was too high for his method to measure.
Our Eyes Receive Rays
Philoponus correctly took the view that our eyes do not send out rays to allow us to see, they receive rays. He believed air allows colors to pass through it without becoming colored itself. He supported this with his observation that a stained-glass window casts colors on floors and walls but does not color the air.
Philoponus wrote a short work on the astrolabe much of which was derived from Theon of Alexandria’s book On the Small Astrolabe. The word astrolabe comes from two Greek words meaning ‘to follow the stars.’ It is through Philoponus’ work that today we know something of Theon’s astrolabe work.
The first astrolabe – a spherical astrolabe, more commonly called an armillary sphere – was invented by Eratosthenes in the third century BC.
In the second century BC, Hipparchus realized that using stereographic projection from three dimensions into two dimensions, he could produce a more portable version of the spherical astrolabe – the small astrolabe described by Theon.
Personal Details and The End
John Philoponus is believed to have died in about 570 AD. It is not known whether he married or had children.
About a century after Philoponus’ death, the Christian Church condemned him as a heretic because he advocated tritheism, the doctrine that God was three separate gods – Father, Son, and Holy Spirit. Philoponus’ name became notorious and his scientific work was less trusted than otherwise. However, his work did not disappear.
Philoponus’ concept of a force impressed into the projectile re-emerged with scientists such as Avicenna in the eleventh-century Islamic world. However, Philoponus’ great conceptual breakthrough that the heavenly bodies also move because of an impressed force were ignored by Muslim scholars. This might have been because Philoponus wrote about this concept in his firmly Christian work De opificio mundi.
In the early 1300s, Nikephoros Kallistos Xanthopoulos, the Greek Church historian, wrote his great work Historia Ecclesiastica. In it he devoted a chapter to Philoponus in which he said Philoponus’ religious beliefs were improper, but his scientific work was masterful and still studied.
Also in the 1300s, Jean Buridan, who lectured on Aristotle at the University of Paris produced an impetus theory, possibly inspired by Philoponus’ writings. 800 years after Philoponus and 300 years before Galileo (scientific progress in the Middle Ages was painfully slow!) Buridan, agreeing that the force of motion was impressed into the projectile rather than the air, suggested that the impetus of the projectile was eroded by air resistance and gravity.
Philoponus’ writings planted intellectual seeds in successive generations of natural philosophers. The seeds finally flowered fully in the early 17th century when Galileo liberated science from Aristotle’s chains.
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John Philoponus’ Criticism of Aristotle’s Theory of Aether
Walter de Gruyter, 1988
Richard Sorabji et al.
Philoponus and the Rejection of Aristotelian Science
University of London Institute of Classical Studies, April 2010
William. A. Wallace
Prelude to Galileo: Essays on Medieval and Sixteenth-Century Sources of Galileo’s Thought
Springer Science & Business Media, Dec 2012
3D representation of heavens from Earth and stereographic projection of 3D to 2D by Michele Invernizzi, DensityDesign Research Lab, under the Creative Commons Attribution-Share Alike 4.0 International license.
Exploded view of small astrolabe by Elrond under the Creative Commons Attribution-Share Alike 4.0 International license.