Hans Peter Broedel

This clerical embrace of Aristotle had a number of interesting consequences relevant to the development of medieval science. First, Aristotle believed that all knowledge originated in sense experience, which was a major departure from the epistemology (way of knowing) of St. Augustine and the earlier middle ages.  High medieval churchmen certainly did not deny that direct revelation from God was possible, but insisted that it was unusual, and so the best way to understand God was to understand what we could perceive directly, that is, the natural world.  As the theologian, Hugh of St. Victor put it in the twelfth century, “The whole of the sensible world is like a kind of book written by the finger of God… and each particular creature is somewhat like a figure, not invented by human decision, but instituted by the divine will to manifest the invisible things of God’s wisdom.”1  The work of natural philosophy , then, was to decode the book of nature, so to speak, in order to reveal the hidden hand of God.  This led medieval scholars to study animals and plants, stars and planets, water, fire, and all manner of natural phenomenon.  Further, although understanding God was the ultimate goal, his creation was assumed to follow rules that did not require His constant intervention, and so, like Aristotle, they described nature in what we would call “natural” terms.  Miracles could, of course, still happen, but that was the provenance of theologians; natural philosophy dealt with nature, not with God directly.

In this way, medieval scholars were encouraged to explore the natural world, to build upon the work of their classical predecessors, but at the same time to acknowledge that the wonder of nature was a testament to the glory of God.  Although they worked within an Aristotelian cosmos, and accepted as complete truth the great Philosopher’s (Aristotle’s) basic assumptions, they also recognized that their own work surpassed that of the ancients, both in its Christianity and in its capacity to build upon the achievements of the past.  Bernard of Chartres, a twelfth-century philosopher and theologian, put it neatly when he observed that the scholars of his day were like “dwarves on the shoulders of giants and thus we see more and farther than they did.”2  This meant that when necessary they were even prepared to try to correct the great Philosopher’s mistakes.

Aristotle explained most things quite well, but his rules of motion were an exception.  Aristotle dictated that inanimate objects move naturally to their proper sphere, but, otherwise, they only move if they are pushed by something else.  This makes sense at first: if I want to move a piano, I’m going to have to push it, and once I stop, so will the piano.  But what about an arrow?  The motive force of the bow is removed when the arrow leaves the string, but the arrow clearly continues to move.  Aristotle’s answer, like the rest of his physics, is extremely complicated, but he argues in effect that the force of the bow not only moves the arrow but the air around it, and that the air continues to push the arrow proportionally to the force that initially sets it in motion.  This seems pretty ridiculous on its face, but medieval scholars had a serious vested interest in maintaining the integrity of the Aristotelian cosmos, and so they began to investigate motion diligently.  One of main ways that their approached differed from the Aristotle’s was that they tried to describe motion mathematically.  For Aristotle, this was a huge mistake, because numbers were completely abstract concepts that exist only in the mind, not in nature.  To describe nature in such “unnatural” terms was invalid.  Similarly, Aristotle would have rejected what would later come to be called “experiments,” because they artificially constrained nature to behave in unnatural ways.  Rather, the Aristotelian scientist observed nature passively, recording what it did, not what it was made to do.

Yet, in an attempt to salvage his cosmos, medieval natural philosophers rejected Aristotle’s methodological criticism, and tried to figure out exactly how projectiles move.  They failed, unsurprisingly, because they could not abandon the basic principles of the Aristotelian cosmos, but their failures nonetheless foreshadowed  the mathematical modeling that was such an essential part of the new science of the sixteenth and seventeenth centuries.3  In the early fourteenth century, a series of remarkable scholastic physicists at Oxford’s Merton College, sometimes dubbed the Merton Calculators, tried to solve to the problems of motion using only mathematics and what we might call “thought experiments.”  Many of their results, in retrospect, proved quite wrong, but they did show conclusively that mathematics could be used to model natural phenomena, and eventually expounded what we now call “the mean speed theorem” (that a moving body undergoing continuous acceleration will travel a distance in a given time exactly equal to that of a body moving at a constant speed equal to the mean speed of the accelerating body).

Equally significant, the community of medieval scholars built on this work.  So, a few years after the Merton Calculators, Nichole Oresme (d. 1382), bishop of Orleans, developed a geometric proof of the Merton theorem that provides us with  one of the very eariiest examples of the use of a graph to model a mathematical function.4  (A purely mathematical proof of the theorem would await the development of the calculus.)  Oresme, by the way, was also notable for proposing that the earth revolved.  He remained committed to the notion that the earth was at the center of the cosmos, but argued that it was more economical to suggest that the earth turned while the surrounding heavens stood still.  He systematically replied to various counterarguments, including suggesting that the reason that an arrow shot straight upwards comes straight back down, instead of being offset by the motion of a revolving earth, was that the arrow, like the air surrounding it, was spinning at exactly the rate of the earth to begin with.5