 Hi, I'm Peter Adamson and you're listening to the History of Philosophy podcast, brought to you with the support of the Philosophy department at King's College London and the LMU in Munich, online at www.historyoffilosophy.net. Today's episode, A Light That Never Goes Out, Robert Grossetest. Metaphors exercise a stronger influence in philosophy than most philosophers would probably like to admit. Whole political theories have been grounded in the comparison of the state to a human body, as we saw with the Polycraticus of John of Salisbury. Philosophy itself has often been described metaphorically, as when the Stoics drew an analogy between its three parts of ethics, physics, and logic, and the yoke, white, and shell of an egg, or the fruit, trees, and surrounding wall of an orchard. In our own day, misguided parents who discouraged their children from studying philosophy modify this Stoic image, suggesting that philosophy may be more like a natural fertilizer sometimes used in orchards. But no metaphor has been a more constant or influential feature in the history of philosophy than the comparison of knowledge to eyesight. It plays a role not just in major philosophical works like Plato's Republic, but even in our very language. To realize something is to see it, and you can perceive that something is true. To make a nice point in an argument is to offer an observation, while accurate anticipation of future events can be called foresight. The same was true of Ancient Greek. Our word theory comes from the Greek theoria, which means viewing or beholding, but was used by Aristotle to refer to philosophical contemplation. Even the Greek word for intellect, nous, comes from a verb of seeing, noēn. The medievals were enthusiastic users of this metaphor, not least because it went so nicely with another analogy, between God and light. Even this has Greek roots. The Neoplatonists frequently said that their first principle generates all other things like a source of illumination spreading forth its rays. In the Islamic world, the metaphor was central to the illuminationist philosophy of the 12th century thinker Sukhravarti. Christians could read in the Book of John that God is light, and this was taken up and developed by patristic authors like Augustine and the Pseudo-Dionysius. The next step is considerably more obvious than putting together peanut butter and chocolate to get Reese's peanut butter cups. If God is light, and knowledge is vision, then perhaps God is the light that enables us to have knowledge. That in a peanut shell is what has come to be called the medieval theory of divine illumination. It is based on one of Augustine's favorite metaphors, and also captures his idea that we can only achieve knowledge thanks to Christ's presence as an inner teacher in our souls. So naturally enough, it is often seen, there's that visual metaphor again, as a distinctively Augustinian aspect of 13th century medieval thought. After all these comparisons though, comes a contrast, between this Augustinian divine illumination model and an Aristotelian theory of knowledge, according to which humans come to know things through sense experience. Aristotle proposed a bottom-up epistemology building on sensation, whereas the Augustinians insisted that something as exalted as knowledge can only come from the very top, that is from God. The implications of the dispute are far-reaching. For instance, the Augustinian theory makes it sound like the best way to achieve knowledge might be to withdraw from the world and meditate, or contemplate, in the hopes of opening oneself to the divine light. The Aristotelian model would instead encourage us to go out into the world and investigate, in order to activate the mind's potential for understanding. But as so often, this neat contrast gets messier the more closely you look at it. Aristotle himself used the light metaphor in one of his most influential bits of writing, where he compares a mysterious, unidentified maker-intellect to the light that makes eyesight possible. In antiquity, Alexander of Aphrodisias, no Platonist, never mind Augustinian, identified this intellect with God. Meanwhile, we find pioneers of the divine illumination theory emphasizing the need for what they call experimentum, not experiment exactly, but sensory experience of the physical world around us. In this episode I'll be looking at such a pioneer, Robert Grossetest. He gives light a central role in his philosophy, not only in explaining knowledge, but also in setting out a breathtakingly original cosmology, which involves some rather illuminating suggestions about mathematics. Most of the major philosophers of 13th century medieval Europe were active in the second half of the century. Like Philip the Chancellor, Robert Grossetest is an exception. He died in 1253, having served as Chancellor of the University of Oxford. These were good decades for Chancellors, before being consecrated as Bishop of Lincoln in 1235. On that basis, you might conclude that we're dealing here with a distinctively English thinker. Certainly, there's no denying that Grossetest is an important figure for the development of philosophy at Oxford. There is a scholarly controversy lurking here though, since some have proposed that Grossetest must have received an intellectual formation in Paris. Wherever he received his initial education, he wasn't content to stop learning. In the early 1230s, when he was already into his 60s, he decided to learn Greek. His motivation was probably, above all, theological. He wished to read the New Testament in the original. He was able to draw on the assistance of Greek-speaking scholars, who had begun to travel in Europe more frequently following the crusader's sack of Constantinople in 1204. Remarkably, Grossetest not only mastered the language, but went on to execute translations of key philosophical works, including Aristotle's Nicomachean Ethics and On the Heavens. He also consulted and translated Greek commentaries on these works from the Late Ancient and Byzantine periods. This aspect of Grossetest's career is apt to remind us of the 9th century translator and philosopher, Eriugena. They even both translated writings of Dionysius. Grossetest's light-based speculations also tend to make him seem a distinctively Platonist thinker not unlike Eriugena. But Grossetest was doing philosophy in the 13th century, and that almost always meant studying the works of Aristotle. He was no exception. Grossetest's understanding of knowledge as involving illumination from God features in his comments on that key work of Aristotelian epistemology and scientific theory, the posterior analytics. Confronted with Aristotle's pronouncement that we have demonstrative knowledge of something only when we know its cause, Grossetest says that he couldn't agree more. He's also happy to sign up to the doctrine that knowledge must be universal in character. The illuminationist theory comes in when he explains how we are able to have such knowledge. Universals, Grossetest explains, appear at numerous levels of reality. They exist in physical things around us as their forms, and that sound you hear is Peter Abelard turning in his grave. Universals exist in the heavens and in the minds of angels, and finally, and most importantly, they exist in God's mind as the eternal, paradigmatic reasons of things. That other sound you hear is Aristotle turning in his grave, as Grossetest says that the whole theory of demonstrative knowledge can only work if we factor in these rather Platonist-sounding divine ideas. They are not only the source of all universality, but also the true causes of all created things. And you did say, didn't you Aristotle, that we can only know things by knowing their causes? But we shouldn't get carried away in emphasizing the perversely anti-Aristotelian nature of Grossetest's commentary on Aristotle. He is totally committed to Aristotle's idea that humans cannot reach knowledge without sensation. Admittedly, he does have a distinctively Christian reason for this, which is that in our fallen state, we cannot just know things in God's light. Instead, we must engage in laborious empirical study to make up for our weakness as knowers. But as Grossetest puts it, knowledge comes to us via the senses, but not from the senses. For no knowledge would be possible without the illumination provided by the divine light. Even people who know nothing of God are, unbeknownst to themselves, grasping truths in the light of God's reasons. I should head off a common misconception at this point, which is that theories of divine illumination always involve simple acts of awareness, inexpressible in language and perhaps even mystical in character. This is certainly not Grossetest's view. Though he sometimes talks as if we grasp simple essences in God's light, he more usually mentions it to explain how we grasp complex propositions. Otherwise, he could hardly integrate this theory into a discussion of Aristotelian demonstration. The contribution of divine illumination is to ground the certainty of syllogistic arguments. When we reach a demonstration, our minds come a bit closer to God's own mind, which of course permanently understands the causes or reasons of everything. It is however important to him, as it would be for some more mystical authors, to assert that in the ideal case, we have a direct illumination from God. This is the situation of the blessed in the afterlife. They will enjoy a grasp of God without mediation, as Grossetest puts it. On this point, he is willing to voice a rare explicit disagreement with Dionysius, whose relentless emphasis on divine transcendence didn't allow for such a direct vision of God. Instead, Dionysius had proposed that souls in heaven get only a representation of what God may be like in himself, like people who didn't get to go to Woodstock and have to content themselves with watching documentaries about it. Interesting though Grossetest's translations and commentaries are, it is his little treatise On Light that is most likely to, as the Woodstock generation would have put it, totally blow your mind. It's a brief work, only a few pages long, but these are among the most innovative pages of philosophy written in the first half of the 13th century. As you will not be surprised to hear, On Light deals with the topic of light. It treats this topic, not in the context of the study of human eyesight, or for that matter human knowledge, though Grossetest tackled both issues elsewhere. Instead, his gist here is that the entire cosmos is quite literally formed from light. This light begins at a single point, and then disperses itself through space, as far as the outermost sphere of heaven, which is the most pure and perfect body, primary matter which has received primary form, the primary form being light itself. At this outermost limit of the universe, body is at its most subtle. The universe contains a series of concentric celestial spheres, and below it the most dense of bodies, the four Aristotelian elements, and ultimately earth. It's obvious that Grossetest is here trying to understand the Bible's statement that God began to create by saying, let there be light. But that isn't the only source for his spectacular proposals. Firstly, there is his understanding of the mechanics of light. It's crucial to his theory that each point of light has a natural tendency to spread out in all directions. This principle was first set out in the Arabic tradition by al-Kindi, and then taken up by the great Ibn al-Haytham, the first scientist to come close to a true understanding of human eyesight. Grossetest probably did not know Ibn al-Haytham, who was translated into Latin too late for him to read, but he read optical works by al-Kindi as well as a treatise by him called On Rays, which tries to explain a wide range of physical and magical phenomena by appealing to the influence of rays. Another figure lurking in the background is Ibn Gabirol. Grossetest takes from Ibn Gabirol the idea that light is form, and that the extension of form carries matter along with it. For, as Grossetest states, not once, but twice, the form of light cannot exist without matter. This is fundamental, because it explains why a material universe should arise from the natural dispersion of light. As far as modern scholars have been able to discover though, Grossetest's use of mathematics to explain this dispersion seems to be all his own. His idea is that as it extends outwards, light is multiplied into three dimensions. This requires not just finite, but infinite expansion. Not because an infinite amount of space needs to be occupied. To the contrary, Grossetest retains the traditional cosmology of Aristotle, according to which the universe is finite and spherical in form. It is rather because a point can only become a line, a line a surface, and a surface three-dimensional, through an infinite multiplication. He in fact considers a point to be a part of a line, and smaller angles to be parts of a larger angle. As light is multiplied through the three dimensions, ratios arise between different spatial magnitudes. On standard medieval assumptions about infinity this is clearly impossible. One infinity cannot stand in a definite ratio to another by being its double or triple. Grossetest rejects these standard assumptions though. He gives the example of the numbers, which are twice as numerous as the even numbers. On the modern-day understanding of infinity, Grossetest is wrong about that particular example. But if he's lost the battle, he's winning the war, because the modern-day mathematician would say that he's right to recognize some infinities as larger than others. This is what is meant by saying that one infinite set has a larger cardinality than another. For instance, the irrational numbers compared to the irrational numbers. So score one point for Grossetest, and given his views on the infinite self-multiplication of points, that could be a pretty big victory. But of course, for him, the mathematics is only a means to the end of explaining God's creation. Actually, he never mentions God in On Light, but it has been plausibly suggested that Grossetest was led to ponder the notion of infinity by reflecting on the boundlessness of God's mind. We can also see him as trying to capture what other thinkers of his time were approaching through the idea of transcendentals. If God is a Light, and all things in creation are somehow fashioned from Light, then Light is a unifying principle for all things. Light is metaphysically fundamental and also required for knowledge, so that we can compare it to the transcendentals of Being and Truth. Grossetest himself connects Light to another transcendental, namely, beauty. Just as Light alone has the natural tendency to diffuse itself, so Light alone is intrinsically beautiful. This is why the most beautiful body in the universe is the outermost celestial sphere, which is a pure fusion of Light with primary matter. Given his fascination with Light, and his emphasis on empirical observation of the world around us, it would be a bit disappointing if Grossetest had nothing to say about actual eyesight. So it's with relief that we can turn finally to his remarks on the rainbow, one of several themes in optics that he discusses in his works. He wrote a treatise dedicated specifically to the rainbow, in which he rejected Aristotle's account of this phenomenon. He made a real advance by proposing that the rainbow effect is created by the refraction of sunlight through a cloud. This idea was adopted and adapted a generation or so later by Roger Bacon. Bacon agreed that Light is being refracted to produce a rainbow, but not by the cloud as a whole. Rather, it is the individual droplets of water in the cloud, with Light bouncing off them as if from innumerable tiny mirrors. Grossetest's explanation of the rainbow tells us something about his more general approach to science. On the one hand, the science of optics is grounded in pure mathematics. By thinking of rays of light and their reflections as lines and angles, mathematicians since Euclid had long been applying the tools of geometry to mirrors and shadows. On the other hand, a mirror reflection, the casting of a shadow or a rainbow, is a physical phenomenon. In talking about the rainbow, Grossetest, and after him Roger Bacon, are not content just to provide a geometrical model that could explain what we see. They invoke empirical evidence for and against a given model. It's on this basis that Grossetest argues for refraction of light rays rather than mirror reflection. Still, there has been quite a bit of controversy about how much credit we should give Grossetest for developing new techniques in empirical science. At one end of the spectrum, A.C. Crombie published a book in the 1950s with the provocative title, Robert Grossetest and the Origins of Experimental Science. At the other end, the leading scholar of his thought James McAvoy has tartly commented that Grossetest was too bookish to be bothered experimenting. While that may underrate his contributions in empirical science, it would be going too far to say that Grossetest did anything so grand as invent or anticipate the experimental scientific method. His idea is rather that, with God's light making the world intelligible to us, we can reach scientific understanding simply by observing. As I said earlier, this is the real meaning of his term expedimentum. It should be translated experience, or observation, not experiment. On the other hand, Grossetest was certainly an eager proponent of the use of mathematics in physics, and in this sense he does foreshadow later scientific developments. But it would be rash to draw general conclusions about experience and science in the 13th century without first examining Roger Bacon. He was one of Grossetest's greatest admirers and lavished praise on the Bishop of Lincoln. Yet Bacon tends to outshine Grossetest in the minds of most historians. Find out why next time on The History of Philosophy Without Any Gaps.