Oxford University, and specifically Merton College, was a site of intense intellectual ferment in the middle of the Medieval Period around the time of Chaucer. A string of natural philosophers, today called the Oxford Scholars or the Oxford Calculators, began to explore early ideas of motion, taking the first bold steps beyond Aristotle. They were the first “physicists” (although that term would not be used until our own time) and laid the foundation upon which Galileo would build the first algebraic law of physics.

It is hard to imagine today what it was like doing mathematical physics in the fourteenth century. Mathematical symbolism did not exist in any form. Nothing stood for anything else, as we routinely use in algebra, and there were no equations, only proportions.

Thomas Bradwardine (1290 – 1349) was the first of the Scholars, arriving at Oxford around 1320. He came from a moderately wealthy family from Sussex on the southern coast of England not far from where the Anglo Saxon king Harold lost his kingdom and his life at the Battle of Hastings. The life of a scholar was not lucrative, so Bradwardine supported himself mainly through the royal patronage of Edward III, for whom he was chaplain and confessor during Edward’s campaigns in France, eventually becoming the Archbishop of Canterbury, although he died of the plague returning from Avignon before he could take up the position. When not campaigning or playing courtier, Bradwardine found time to develop a broad-ranging program of study that spanned from logic and theology to physics.

Merton College, Oxford (attribution: Andrew Shiva / Wikipedia)

Bradwardine began a reanalysis of an apparent paradox that stemmed from Aristotle’s theory of motion. As anyone with experience pushing a heavy box across a floor knows, the box does not move until sufficient force is applied. Today we say that the applied force must exceed the static force of friction. However, this everyday experience is at odds with Aristotle’s theory that placed motion in inverse proportion to the resistance. In this theory, only an infinite resistance could cause zero motion, yet the box does eventually move if enough force is applied. Bradwardine sought to resolve this paradox. Within the scholastic tradition, Aristotle was always assumed to have understood the truth, even if fourteenth-century scholars could not understand it themselves. Therefore, Bradwardine constructed a mathematical “explanation” that could preserve Aristotle’s theory of proportion while still accounting for the fact that the box does not move.

It is hard to imagine today what it was like doing mathematical physics in the fourteenth century. Mathematical symbolism did not exist in any form. Nothing stood for anything else, as we routinely use in algebra, and there were no equations, only proportions. The introduction of algebra into Europe through Arabic texts was a hundred years away. Not even Euclid or Archimedes had been rediscovered by Bradwardine’s day, so all he had to work with was Pythagorean theory of ratios and positive numbers—even negative numbers did not exist—and the only mathematical tools at his disposal were logic and language. Nonetheless, armed only with these sparse tools, Bradwardine constructed a verbal argument that the proportion of impressed force to resistance must itself be as a proportionality between speeds. As awkward as this sounds in words, it is a first intuitive step towards the concept of an exponential relationship—a power law. In Bradwardine’s rule for motion, the relationships among force, resistance and speed were like compounding interest on a loan. Bradwardine’s rule is not correct physics, because the exponential function is never zero, and because motion does not grow exponentially, but it did introduce the intuitive idea that a physical process could be nonlinear (using our modern terminology), changing from small effects to large effects *disproportionate* to the change in the cause. Therefore, the importance of Bradwardine was more his approach than his result. He applied mathematical reasoning to a problem of kinetics and set the stage for mathematical science.

A few years after Bradwardine had devised his rule of motion, a young mathematician named William Heytesbury (1313—1373) arrived as a fellow of Merton College. In the years that they overlapped at Oxford, one can only speculate what was transmitted from the senior to the junior fellow, but by 1335 Heytesbury had constructed a theory of continuous magnitudes and their continuous changes that included motion as a subset. The concept of the continuum had been a great challenge for Aristotelian theory, leading to many paradoxes or sophisms, like Zeno’s paradox that supposedly proved the impossibility of motion. Heytesbury shrewdly recognized that the problem was the ill-defined idea of instantaneous rate of change.

Heytesbury was just as handicapped as Bradwardine in his lack of mathematical notation, but worse, he was handicapped by the Aristotelian injunction against taking ratios of unlike qualities. According to Aristotle, proportions must only be taken of like qualities, such as one linear length to another, or one mass to another. To take a proportion of a mass to a length was nonsense. Today we call it “division” (more accurately a distribution), and mass divided by length is a linear mass density. Therefore, because speed is distance divided by time, no such ratio was possible in Heytesbury’s day because distance and time are unlike qualities. Heytesbury ingeniously got around this restriction by considering the linear distances covered by two moving objects *in equal times*. The resulting linear distances were similar qualities and could thereby be related by proportion. The ratio of the distances become the ratio of speeds, even though speed itself could not be defined directly. This was a first step, a new tool. Using this conceit, Heytesbury was able to go much farther, to grapple with the problem of nonuniform motion and hence the more general concept of instantaneous speed.

In the language of calculus (developed by Newton and Leibniz 300 years later), instantaneous speed is a ratio of an element of length to an element of time in the limit as the elements vanish uniformly. In the language of Heytesbury (Latin), instantaneous speed is simply the average speed between two neighboring speeds (still working with ratios of distances traversed in equal times). And those neighboring speeds are similarly the averages of *their* neighbors, until one reaches zero speed on one end and final speed on the other. Heytesbury called this kind of motion *difform* as opposed to *uniform* motion.

A special case of difform motion was uniformly difform motion—uniform acceleration. Acceleration was completely outside the grasp of Aristotelian philosophers, even Heytesbury, but he could imagine a speed that changed uniformly. This requires that the *extra* distance travelled during the succeeding time unit relative to the distance travelled during the current time unit has a fixed value. He then showed, without equations, using only his math-like language, that if a form changes uniformly in time (constant rate of change) then the average value of the form over a fixed time is equal to the average of the initial and final values. This work had a tremendous importance, not only for the history of mathematics, but also for the history of physics, because when the form in question is speed, then this represents the discovery of the mean speed theorem for the case of uniform acceleration. The mean speed theorem is often attributed to Galileo, who proved the theorem as part of his law of fall, and he deserves the attribution because there is an important difference in context. Heytesbury was not a scientist nor even a natural philosopher. He was a logician interested in sophisms that arose in discussions of Aristotle. The real purpose of Heytesbury’s analysis was to show that paradoxes like that of Zeno could be resolved within the Aristotelian system. He certainly was not thinking of falling bodies, whereas Galileo was.

Not long after Heytesbury demonstrated the mean speed theorem, he was joined at Merton College by yet another young fellow, Richard Swineshead (fl. 1340-1354). Bradwardine was already gone, but his reputation survived, as well as his memory, in the person of Heytesbury, and Swineshead became another member in the tradition of the Merton Scholars. He was perhaps the most adept at mathematics of the three, and he published several monumental treatises that mathematically expanded upon both Bradwardine and Heytesbury, systematizing their results and disseminating them in published accounts that spread across scholastic Europe—all still without formulas, symbols or equations. For these works, he became known as The Calculator. By consolidating and documenting the work of the Oxford Scholars, his influence on the subsequent history of thought was considerable, as he was widely read by later mathematicians, including Leibniz, who had a copy of Heytesbury in his personal library.

( To read more about the Oxford Scholars, and their connections with members of their contemporaries in Paris, see Chapter 3 of Galileo Unbound (Oxford University Press, 2018).)

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