Overcoming folk-physics: the case of projectile motion for Aristotle, John Philoponus, Ibn-Sina & Galileo

A few years ago, I wrote about the importance of pairing tools and problems in science. Not selecting the best tool for the job, but adjusting both your problem and your method to form the best pair. There, I made the distinction between endogenous and exogenous questions. A question is endogenous to a field if it is motivated by the existing tools developed for the field or slight extensions of them. A question is exogenous if motivated by frameworks or concerns external to the field. Usually, such an external motivating framework is accepted uncritically with the most common culprits being the unarticulated ‘intuitive’ and ‘natural’ folk theories forced on us by our everyday experiences.

Sometimes a great amount of scientific or technological progress can be had from overcoming our reliance on a folk-theory. A classic examples of this would be the development of inertia and momentum in physics. In this post, I want to sketch a geneology of this transition to make the notion of endogenous vs exogenous questions a bit more precise.

How was the folk-physics of projectile motion abandoned?

In the process, I’ll get to touch briefly on two more recent threads on TheEGG: The elimination of the ontological division between artificial and natural motion (that was essential groundwork for Darwin’s later elimination of the division between artificial and natural processes) and the extraction and formalization of the tacit knowledge underlying a craft.

Let’s start with the natural and artificial distinction by looking at a domain where it rarely exists: matter. Even for the ancient Greeks, it was clear that both natural objects and artificial objects were made of the same stuff. This was clear because those artificial objects were often made from natural matter. For Aristotle, the form would change to the agency of the artificer, but the matter would remain.

But this is not how he saw motion.

In the everyday experience of an ancient Greek, there were clearly two different kinds of motion. There were objects like stones, water, etc; and there were animals. One seemed to be reacting, returning to a ‘natural’ state. The other seemed able to self-generation motion — to create artificial motion. There is no reason to believe that the two types of motions should have similar explanations. There was a clear division between natural motion and artificial (or animal) motion. This was the folk-physics.

For Aristotle, the continued motion of a natural object would require the continued action of a force (from a animal). This is largely coherent with our every-day experience: you stop pushing a box and it stops moving. If you were pushing it uphill then it might even try to return to its natural position at the bottom of the hill.

But what of projectile motion? When I throw a stone up in the air, it doesn’t instantly stop moving and try to return to its natural place in the earth once my hand stops pushing it. In the framework of folk-physics, the question becomes: “what keeps pushing the stone when I let go of it in a throw?” Aristotle had an answer: the air. From this, the philosopher could explain why the stone didn’t go as far before changing its motion in water, and that it is impossible for a vacuum to exist (resolving an important philosophical question at the time). Although I don’t have a good way of making sense of this in the natural vs artificial distinction — and that makes me suspect that the question of pushing was a folk-question and not forced by the formal distinction between natural and artificial notion.

In the 6th century — about 850 years after Aristotle — John Philoponus started to depart from the philosopher’s answer, but you can see Philoponus struggling against the framework of folk-physics without breaking free. For Philoponus, it was the stone that kept pushing the stone. Our handling of the stone transmitted part of our animal spirit to it, granting it — like other animals — motile power. Giving the natural some artificial. However, since stones aren’t animals, the animal spirit does not naturally reside there so it would flee the stone. Once the granted motile power is gone, the stone would try to return to its natural place in the earth by falling down. Yaḥyā al-Naḥwī — as John Philoponus was known in the Muslim world — approaches our high-school understanding of momentum and starts to formally use the natural vs artificial motion methodology, but is still clearly trying to answer the wrong question forced on him by folk-physics.

It is not until Ibn-Sīnā’s — or Avicenna’s, as he is known in the Latin tradition — Book of Healing published around 1027 that we start to get close to what we’d now call Newton’s first law on motion (for a more comprehensive history of this, see Sayili, A. (1987). Ibn Sīnā and Buridan on the Motion of the Projectile. Annals of the New York Academy of Sciences, 500(1): 477-482). Ibn Sina builds on Yaḥyā al-Naḥwī’s account by accepting that a motile power was acquired by the stone from the thrower, but this spirit does not leave on its own account as unnatural, instead, this impressed virtue is dissipated through the influence of other agents such as the air. At this point, the inquiry can be said to have transformed the question from the ancient “what keeps pushing the stone?” to the modern “what causes the stone to stop moving?”. In the process, little to nothing has been learned about the problem-domain of projectile motion, but a new framework for thought was erected.

The final act comes with Galileo and formalization of Techne. In the cases of the philosophers discussed above, the motivation for projectile motion is detached from practice. It is a sideshow to more important metaphysical questions (like the possibility of vacuum). Galileo, however, was a mathematician. And in 16th/17th century Italy — unoike today — a mathematician was a highly practical person. So for him, projectile motion was not a sideshow, but an important problem in formalizing a craft.

No stone thrower, javelineer, or archer had asked Aristotle, Philoponus, or Ibn-Sina on how to handle their weapon (although maybe some siege engineers asked Archimedes; but I’ll shelve that tangent for now). Instead, the was-craftsmen had an implicit an intuitive craft that let them deliver their projectiles accurately. These masters of war-craft would not be able to describe that projectile motion as parabolic, nor reply to a range and wind measurement with an explicit statement about the angle and force of release. But they would hit the target.

But that implicit knowledge starts to fail for the gunner. While the cannon becomes more important to European warfare in the 16th/17th century, its shot is expensive and slow to aim and reload. So the gunner, unlike the previous masters of projectile motion, does need to reply to a range and wind measurement with an explicit statement about the elevation of the barrel and the amount of gunpowder. With philosophers occupied elsewhere, here enters the mathematician. It now becomes important for Galileo to formalize the craft of projectile motion.

Now the question of projectile motion is shaped much more by the tools: both physical and theoretical. And Galileo can give us our modern answer. But instead of using actual cannons — an expensive and dangerous idea — Galileo used an inclined plane: a tool he developed earlier for the study of uniformly accelerating motion. He would roll an inked ball down an inclined plane set on his table (with a curved end to transform all motion into horizontal) and record where it landed on the floor. From this, he could vary the ball’s initial velocity (another important conceptual and practical tool) and vertical drop to determine that the projectile’s path was parabolic.

In the process, he also blurred the distinction between artifical motion (the projectile) and natural motion (a ball trying to get to the bottom of the inclined plane). This allowed us to eventually eliminate the ancient distinction between natural and artificial motion as two different kinds.