THE NEW EUROPEAN PHILOSOPHY OF NATURE WOULD EMERGE IN the universities of Paris and Oxford, where the intellectual ferment of the diverse student body produced revolutionary ideas that, as we have seen, occasionally brought down on them the heavy hand of the Church, though never for long. The intellectual revival of western Europe was now unstoppable, though even the rebels were still rooted in the ideas of Aristotle, revised so as not to run counter to Catholic dogma, as in the Thomistic philosophy I studied as an undergraduate.
The new philosophy of nature would be based on theory and experiment, as well as mathematics, though at this stage scientists in Europe had basically the same equipment as did the Greeks of the Hellenistic period, when Ptolemy used an astrolabe for his astronomical observations and simple optical instruments for his study of light, so that their experimentation was restricted to optics. Nevertheless, their studies of light, which they extended to acoustics, the science of sound, produced results that in some cases went beyond the level achieved by their Greek and Arab predecessors, setting the stage for the emergence of modern optics in the seventeenth century. They also went beyond their predecessors in the study of the refraction of light by lenses, leading to the invention of the telescope and the microscope.
One of the most influential figures in the rise of the new European philosophy of nature was Robert Grosseteste (c. 1175–1253). His biographer, A. C. Crombie, calls him “the real founder of the tradition of scientific thought in medieval Oxford, and in some ways, of the modern English intellectual tradition.”
Born of humble parentage in Stradbroke in Suffolk, England, he was educated at the cathedral school in Lincoln and then at the University of Oxford. Grosseteste was in the household of William de Vere, bishop of Hereford, by 1198, when a reference by Gerald of Wales suggests that he may have had some competence in both law and medicine, with a manifold learning “built upon the sure foundation of the liberal arts and an abundant knowledge of literature.”
This comment is substantiated by what is probably Grosseteste’s first work, De artibus liberalibus. In the introduction he describes how the seven liberal arts acted as a purgative of errors and gave direction to the mind. His treatment of music is particularly interesting, since for him the laws of harmony applied not only to the human voice and musical instruments but also to the movement of the celestial bodies, the composition of bodies made up of the four terrestrial elements, and the harmonic relation between body and soul in man. He also wrote a related essay entitled De generatione sonorum, in which he describes sound as a vibratory motion propagated from the sounding body through the diaphragm of the ear, whose motion arouses a sensation in the soul.
After that Grosseteste probably taught in the arts faculty at Oxford until 1209, when the masters and scholars of the university were dispersed for five years because of a particularly violent student riot in the town. During those years he received a master’s degree in theology, probably at the University of Paris. At some time in the period 1209–1214 he was appointed magister scholarum, or chancellor of the University of Oxford, probably the first, or one of the first, to hold this office. He also would have lectured on theology, while apparently beginning his own study of Greek. When the first Franciscan monks came to Oxford in 1224, Grosseteste was appointed as their reader, and he directed their interests toward mathematics and natural science as well as the study of the Bible and languages. He finally left the university in 1235 when he was appointed bishop of Lincoln, his jurisdiction including Oxford and its schools. During his episcopate he attended the First Council of Lyons in 1245. He died on December 9, 1253, in Buckden in Buckinghamshire, and he was buried in the cathedral at Lincoln.
Grosseteste’s writings are divided into two periods, his chancellorship of Oxford and his tenure as bishop of Lincoln. His writings in the first period include his commentaries on Aristotle and the Bible and most of his independent treatises. Those in the second period are principally his translations from the Greek: Aristotle’s Nicomachean Ethics and On the Heavens, the latter along with his version of the commentary by Simplicius, as well as several theological works. He brought to Lincoln scholars who knew Greek to assist him in his translations. He also arranged for a translation of the Psalms to be made from the Hebrew, and he seems to have learned something of that language.
The commentaries that Grosseteste wrote on Aristotle’s Posterior Analytics and Physics were among the first and most influential interpretations of those works. These two commentaries also presented his theory of science and scientific method, which he put into practice in his own writings, including six works on astronomy and one on calendar reform, as well as treatises entitled The Generation of the Stars, Sound, The Impressions of the Elements, Comets, The Heat of the Sun, Color, The Rainbow, and The Tides, in which he attributed tidal action to the moon.
Grosseteste was the first medieval European scholar to use Aristotle’s methodology of science. Grosseteste’s methodology involved two steps. The first of these was a combination of induction and deduction, which he called resolution and composition, respectively. The second step was what Grosseteste called verification and falsification, a process necessary to distinguish the true cause from other possible causes. He based his use of verification and falsification upon two assumptions about the nature of physical reality. The first of these was the principle of the uniformity of nature, in support of which he quoted Aristotle’s statement that “the same cause, provided that it remains in the same condition, cannot produce anything but the same effect.” The second was the principle of economy, which holds that the best explanation is the simplest, that is, the one with the fewest assumptions, other circumstances being equal. Here again he quoted Aristotle, who said that power from natural agents proceeds in a straight line “because nature operates in the shortest way possible.” Beginning with these assumptions, Grosseteste’s method was to distinguish between possible causes “by experience and reason,” rejecting theories that contradicted either factual evidence or an established theory verified by experience. These ideas led to the scientific method that was used by Galileo and Newton and others to establish the foundations of modern science.
The experiments performed by Grosseteste were principally in optics, studying the reflection of light by plane and curved mirrors and its refraction by lenses as well as a spherical glass container full of water. His experiments on refraction gave him an understanding of color, anticipating some of the discoveries made by Newton and published early in the eighteenth century in his Optiks.
The methodology of Grosseteste also used Aristotle’s procedure of subordinating some sciences to others, such as of astronomy and optics to geometry and of music to arithmetic. As he wrote in one of his Aristotelian commentaries: “With such sciences of which one is under the other, the superior science provides the propter quid [the reason the fact] for that thing of which the inferior science provides the quia [the observed fact].”
According to Grosseteste, it was impossible to understand the physical world without mathematics, which, in my opinion, sets him apart as the first modern physicist. Although mathematics involved a study of abstract quantity, mathematical entities actually existed as quantitative aspects of physical things, for, as he noted, “quantitative disposition are common to all mathematical sciences … and to natural science.” The use of mathematics made it essential to perform measurements that have a quantitative result, though in doing so there was an inescapable inaccuracy, which made all human measurements conventional. But although geometry, for example, could give the “reason for the fact,” in the sense of describing a phenomenon in optics such as reflection of light, it could not provide the efficient and other causes involved. Thus a complete explanation of optical phenomena requires not only geometry, but a knowledge of the physical nature of light that causes it to move as it does in being reflected by a mirror, in which the angle of incidence equals the angle of reflection. That is, mathematics provided only the formal cause; the material and efficient causes were provided by the physical sciences. Thus “the cause of the equality of the two angles made on a mirror by the incident ray and the reflected ray is not a middle term taken from geometry, but is the nature of the geometry generating itself in a straight path.”
Although many of Grosseteste’s ideas were Aristotelian in origin, some of them differed significantly from those of Aristotle. Aristotle, for example, held that all of the celestial bodies were composed of the quintessential element, aether, while Grosseteste believed that the stars were made up of the four terrestrial elements. Also, whereas Aristotle argued that a vacuum was impossible and that space is finite in extent, Grosseteste tried to explain the meaning that could be given to the mathematical concepts of a vacuum and infinite space. He suggested that space “as it was imagined by mathematicians” could be thought of as infinite in extent only because it was not the same as real space.
He explains how mathematics and measurement provided a means of describing natural phenomena. Commenting on Aristotle’s definition of time as “the number of movement in respect of before and after,” he suggested that by using this definition, rates of local motion and other kinds of change could be compared by measuring lengths.
He goes on to say that mathematical physicists must, for practical reasons, measure magnitudes of all kinds by conventional units, such as those of the finger, span, and cubit for length, and “one revolution of the heavens” as a measure of time. Here he is reviving the Platonic notion of “geometricizing nature,” now extended to quantifying even subjective aspects of nature as well as those that are not eternal but change in time. As Crombie has noted, this “foreshadowed in a striking manner a methodological principle on which modern mathematical physics, particularly since the seventeenth century has been based … the principal that, in order to be described in the language of science, ‘subjective’ sensations should be replaced by concepts amenable to mathematical treatment.”
It was Grosseteste’s belief that the study of optics was the key to an understanding of the physical world, a notion that stemmed from his Neoplatonic “Metaphysics of Light.” Following the lead of Saint Augustine, he held that physical light is analogous to the spiritual light by which the mind received true knowledge of the ideal forms that he thought to be the essential principals in the order of nature.
He felt that something was known with complete certitude when the concept of it corresponded to the eternal form existing in God’s mind. In one of his Aristotelian commentaries Grosseteste says of Aristotle’s statement that “the science is more certain and prior which is knowledge at once of the fact and of the reason for the fact.”
According to Grosseteste, in the beginning God created light, the fundamental corporeal form, which multiplied itself infinitely in every direction and in its expansion forced unextended matter into spatial dimensions. He considered light to be the efficient cause of motion and the coordinating principle that gives order and intelligibility to the macrocosm of the created universe as well as governing the interaction between soul and body and the bodily senses in the microcosm of man.
According to his optical theory, light travels in a straight line through the propagation of a series of waves or pulses, and because of its rectilinear motion it can be described geometrically. This was similar to the acoustical theory he presented in his commentary on Aristotle’s Posterior Analytics, where he wrote that “when the sounding body is struck and vibrating, a similar vibration and similar motion must take place in the surrounding contiguous air, and this generation progresses in every direction in straight lines.”
He thought that the same theory, which he called the multiplication of species, could be used to explain the propagation of any disturbance, be it light, sound, heat, mechanical action, or even astrological influence. Thus the study of light was of crucial importance for an understanding of nature. He also believed that light, by which he meant not only visible radiation but the divine emanation as well, was the means by which God created the universe.
The study of optics was divided by Grosseteste into three parts, namely phenomena involving vision, mirrors (catoptrics, or reflection), and lenses (dioptrics, or refraction). He discussed the third part more fully than the other two, noting that it had been “untouched and unknown among us until the present time,” and suggested applications of refraction that in the seventeenth century would be realized through the invention of the telescope and the microscope. “This part of optics,” he wrote, “when well understood, shows us how we may make things a very long distance off appear as if placed very close … and how we may make small things placed at a distance appear any size we want, so that it may be possible for us to read the smallest letters at incredible distances, or to count sand, or grains, or seeds, or any sort of minute objects.”
The reason for this magnification, as he had learned from the optical treatises of Euclid and Ptolemy, was “that the size, position and arrangement according to which a thing is seen depends on the size of the angle through which it is seen and the position and arrangement of the rays, and that a thing is made invisible not by great distance, except by accident, but by the smallness of the angle of vision.” Thus “it is perfectly clear from geometrical reasons how, by means of a transparent medium of known size and shape placed at a known distance from the eye, a thing of known distance and known size will appear according to place, size and position.”
Grosseteste developed a quantitative theory of refraction in an attempt to explain the focusing of light by a “burning-glass,” or spherical lens. According to his law, which is incorrect, when light passes from a dense medium to a rarer one, the refracted ray bisects the angle between the incident ray and the perpendicular to the common surface at the point of entry.
An experiment would have shown Grosseteste that his law of refraction was incorrect, but apparently he never put his law to the test, although it was one of the basic tenets of his scientific method that if a theory was contradicted by observation, it must be abandoned. Crombie explained this curious attitude:
He was, in fact, primarily a methodologist rather than an experimentalist, and also, perhaps, he was too much obsessed with the principle, according to which he believed lux [the essence of visible light] to behave, and with the alleged similarity between refraction and reflection, to arrive at a correct understanding of the problem.
Nevertheless, Grosseteste’s proper application of his scientific method is evident in his treatise on The Rainbow, in which he broke with Aristotelian theory by holding that the phenomenon was due to refracted rather than reflected light. As he writes there, using his notion of the subordination of some sciences to others: “For example, optics falls under geometry, and under optics falls the science concerned with the rays of the sun refracted in a concave watery cloud.” Although his theory of the rainbow was incorrect, he posed the problem in such a way that investigations by those who followed after him approached closer to the true solution through criticizing his efforts.
He held that it was possible to explain qualitative differences in physical powers as stemming from quantitative differences based on geometrical properties of light rays. Thus he tried to explain the intensity of heat and light as due to the concentration of rays, and heat itself as a scattering of molecular parts due to movement caused by radiation. He said the color was “light incorporated with the transparent medium” and that the entire spectrum of colors was produced by the “intension and remission” of three factors, namely the purity of the medium from earthy matter, the brightness of the light, and the quantity of the rays. He concluded: “That the essence of color and a multitude of the same behaves in the said way is manifest not only by reason but also by experiment, to those who know the principle of natural science and of optics deeply and inwardly.… They can show every color they wish to visibly, by art [per artificium].”
There is little in Grosseteste’s philosophical and scientific writings to indicate that he was a Christian bishop, but in his treatise On the Fixity of Motion and Time he differed from the Aristotelian doctrine that the universe is eternal, for that contradicted his belief in God’s creation. His Christian beliefs are also evident in another treatise, On the Order of the Emanation of Things Caused from God, in which he says that he wishes men would cease questioning the biblical account of the creation.
Beside his works on optics, Grosseteste wrote a number of treatises on astronomy. The most important of these was De sphaera, in which he discussed elements of both Aristotelian and Ptolemaic theoretical astronomy. He also wrote of Aristotelian and Ptolemaic astronomy in four treatises on calendar reform, where he used Ptolemy’s system of eccentrics and epicycles to compute the paths of the planets, though he noted that “these modes of celestial motion are possible, according to Aristotle, only in the imagination, and are impossible in nature, because according to him all nine spheres are concentric.”
His works on calendar reform stemmed from his observation that the system that had long been in use—a cycle in which nineteen solar years were equal to 235 lunar months—was in error, as evident from the fact that the moon was never full when the calendar said it should be, which meant that the reckoning of Easter was always wrong. This was due to the inaccuracy both in taking the solar year to be 365.25 days and in having the lunar cycle equal to nineteen years.
To correct the problem, Grosseteste’s plan for calendar reform involved three stages, the first of which was an accurate determination of the solar year. He was aware of three such measurements, that of Hipparchus and Ptolemy, and those of Thabit ibn-Qurra and al-Battani. His analysis of these measurements led him to conclude that al-Battani’s value “agrees best with what we find by observation on the advance of the solstice in our own time.”
The second stage was to find the relationship between the solar year and the mean lunar month, which is slightly more than 29.5 days. Grosseteste, in his first treatise on calendar reform, Canon in kalendarium, had used a quadruple nineteen-year cycle of seventy-six years to compile a set of new-moon tables. In his third treatise, Compotus correctorius, he calculated the error involved in this method and proposed a new and much more accurate cycle, using the Islamic year of twelve lunar months, or slightly more than 354 days. The cycle comprised thirty Islamic years, which together equal 10,631 days, the shortest period in which the cycle of lunations, or new moons, repeats itself. Grosseteste developed a method for combining this Islamic lunar cycle with the Christian solar calendar in order to calculate lunations accurately.
The third stage of Grosseteste’s calendar reform employed his new method for an accurate reckoning of Easter. He writes in his Compotus correctorius that even without an accurate measurement of the solar year, the time of the spring equinox, on which the date of Easter depended, could be determined “by observations with instruments or from verified astronomical tables.”
His treatise On Prognostication discusses astrological influences, along with his theory of tidal action, but he later condemned astrology, calling it a fraud and a delusion of Satan.
Grosseteste’s most renowned disciple was Roger Bacon (c. 1220–1292), who acquired his interest in natural philosophy and mathematics while studying at Oxford. He received an master’s either at Oxford or Paris, in around 1240, after which he lectured at the University of Paris on various works of Aristotle. He returned to Oxford in about 1247, when he met Grosseteste and became a member of his circle, which included Adam Marsh, Grosseteste’s close friend and successor as lecturer at Oxford. Bacon writes of Grosseteste and Marsh in describing his intellectual development.
There have been found some famous men, such as Robert, bishop of Lincoln, Brother Adam of Marsh, and many others, who have known how to utilize the power of mathematics to unfold the causes of all things and to give an explanation of human and Divine phenomena; and the assurance of this fact is to be found in the writings of these great men, as for instance in their works on the impressions [of the elements], on the rainbow, on comets, on the investigation of the places of the world, on celestial things, and on other questions appertaining both to theology and to natural philosophy.
It seems that Bacon became a Franciscan monk in around 1257, and soon afterward he experienced difficulties, probably because of a decree restricting the publication of works outside the order without prior approval. In any event, Pope Clement IV issued a papal mandate on June 22, 1266, asking Bacon for a copy of his philosophical writings. The mandate not only ordered Bacon to send his book but to state “what remedies you think should be applied in these matters which you recently intimated were of such great importance” and “to do this without delay as secretly as you can.”
He eventually sent Clement three works—Opus maius, Opus minus, and Opus tertium—along with a letter proposing a reform of learning in the Church. He maintained that there were two types of experience, one obtained through mystical inspiration and the other through the senses, assisted by instruments and quantified in mathematics. The program of study that he recommended included languages, mathematics, optics, experimental science, and alchemy, followed by metaphysics and moral philosophy, which, under the guidance of theology, would lead to an understanding of nature and through that to knowledge of the Creator.
Within the next few years Bacon wrote three more works, the Communia naturalium, Communia mathematica, and Compendium studii philosophie, the last of which castigated the Franciscan and Dominican orders for their educational practices. At some time between 1277 and 1279 he was condemned and imprisoned in Paris by the Franciscans, possibly because of their censure of heretical Averroist ideas. Nothing further is known of his life until 1292, when he wrote his last work, the Compendium studii theologii.
He appropriated much of Grosseteste’s “Metaphysics of Light” with its “multiplication of species,” as well as his mentor’s emphasis on mathematics, particularly geometry. In his Opus maius Bacon stated that “in the things of the world, as regards their efficient and generating causes, nothing can be known without the power of geometry,” and he also said that “every multiplication is either according to lines, or angles or figures.”
Bacon’s ideas on optics repeat and extend those of Grosseteste, such as in his wave theory of light. Bacon held that light could propagate from one point to another only through a continuous medium and not through a vacuum.
Following Grosseteste, Bacon held that the “multiplication of species” proceeded as a series of pulses propagating through a medium both for light and sound, as he wrote in Opus maius:
For sound is produced because parts of the object struck go out of their natural position, where there follows a trembling of the parts in every direction along with some rarefaction, because the motion of rarefaction is from the center to the circumference, and just as there is generated the first sound with the first tremor, so is there a second sound with the second tremor in a second portion of the air, and a third sound with a tremor in a third portion of the air, and so on.
He pointed out the differences between the propagation of sound, light, and odors:
In the multiplication of sound a threefold temporal succession takes place, no one of which is present in the multiplication of light.… However the multiplication of both as regards itself is successive and requires time. Likewise in the case of odour the transmission is quite different from that of light, and yet the species of both will require time for transmission, for in odour there is a minute evaporation of vapour, which is, in fact, a body diffused in the air to the senses beside the species, which is similarly produced.… But in vision nothing is found except a succession of the multiplication.
He then went on to describe an observation showing that light propagates far more rapidly than sound:
The fact that there is a difference in the transmission of light, sound, and odour can be set forth in another way, for light travels far more quickly in air than the other two. We note in the case of one at a distance striking with a hammer or a staff that we can see the stroke delivered before we hear the strike produced. For we perceive with our vision a second stroke, before the sound of the first stroke reaches the hearing. The same is true of a flash of lightning, which we see before we see the sound of the thunder.
He went beyond Grosseteste in his explanation of vision, to which he paid particular attention because, as he said, “by means of it we search out certain experimental knowledge of all things that are in the heavens and earth.” He gives a better description of the eye and optic nerves than any other Medieval Latin writer. Presenting an optical diagram of the human eye in the Opus maius, he noted, “I shall draw, therefore, a figure in which all of these matters are made as clear as possible, but a full demonstration would require a body fashioned like the eye in all the particulars aforesaid. The eye of a cow, pig or other animal can be used for illustration, if anyone wishes to experiment.” As Crombie wrote of Bacon’s work in this field: “His account of vision was one of the most important written during the Middle Ages and it became a point of departure for seventeenth-century work.” Crombie goes on the quote Bacon in what he calls “a worthy expression of the ideals of the experimental method by one of its founders.”
Bacon followed Grosseteste in suggesting the use of lenses as an aid to vision, which would soon lead to the invention of spectacles. He made a detailed study of the optics of vision and formulated a set of eight rules classifying the properties of convex and concave spherical surface in conjunction with the eye. As he wrote in the Opus maius:
If anyone examines letters or other small objects through the medium of a crystal or glass or some other transparent body placed above the letters, and if it be shaped like the lesser segment of a sphere with the convex side towards the eye, and the eye is in the air, he will see the letters much better and they will appear much larger to him.… For this reason the instrument is useful to old people and people with weak eyes, for they can see any letter however small if magnified enough.
His scientific method was clearly stated in part 6 of the Opus maius, “De scientia experimentali,” which also derives from Grosseteste.
I now wish to unfold the principles of experimental science, since without experience nothing can be sufficiently known. For there are two modes of acquiring knowledge, namely by reasoning or experience. Reasoning draws a conclusion and makes us grant that conclusion, but does not make the conclusion certain, nor does it remove doubt so that the mind may rest on the intuition of truth, unless the mind may discover it by the world of experience.
He then wrote of the “three great prerogatives” of experimental science, the first being “that it investigates by experiment the noble conclusions of all the sciences.” The second, according to Bacon, is that experiment adds new knowledge to existing sciences, and the third is that it creates entirely new areas of science. The new areas included, for example, those in astronomy that would be opened up by the telescope, and in medicine and biology by the microscope, two instruments whose discovery he predicted, along with applications of science in technology that he would write about in his Epistola de secretis operibus artis et naturae et de nullitate magiae. This describes marvelous machines such as self-powered ships, automobiles, airplanes, and submarines:
Machines for navigation can be made without rowers so that the largest ships on rivers or seas will be moved by a single man in charge with greater velocity than if they were full of men. Also cars can be made so that without animals they will move with unbelievable rapidity.… Also flying machines can be constructed so that a man sits in the midst of the machine revolving some engine by which artificial wings are made to flap like a flying bird.… Also a machine can easily be made for walking in the sea and rivers, even to the bottom without danger.
The vital importance of mathematics in science is emphasized by Bacon, who wrote that “no science can be known without mathematics.” He claimed that “in mathematics only are there the most convincing demonstrations through a necessary cause.… Wherefore it is evident that if, in the other sciences, we want to come to certitude without doubt and to truth without error, we must place the foundations of knowledge in mathematics.” He went on to say that Grosseteste and Adam Marsh had followed this method and “if anyone should descend to the particular by applying the power of mathematics to the separate sciences, he would see that nothing magnificent in them can be known without mathematics.”
Writing of the use of mathematics in making known “the things of this world,” Bacon gave as an example astronomy, which “considers the quantity of all things that are included among the celestial and all things that are reduced to quantity.” He said that “by instruments suitable to them and by tables and canons,” one can measure the movements of the celestial bodies and reduce them to rules on which predictions can be based. He also employed Grosseteste’s Compotus to carry on his program of calendar reform, making use of both Aristotle’s physical model of the concentric celestial spheres and Ptolemy’s mathematical method of eccentrics and epicycles.
Bacon also used his scientific method to study the rainbow. He began by examining phenomena similar to rainbows, including the dispersion of colors in crystals, morning dew on grass, water spray, and light refracted by a glass vessel filled with water or seen through cloth or partially closed eyelashes. He then examined the rainbow itself, observing that it always appeared when there was a cloud or mist; that the bow was always opposite the sun, that the center of the bow was always in a straight line with the observer and the sun, and that there was a definite relation between the altitude of the rainbow and the sun. Reporting on his observation of a rainbow with an astrolabe, he noted: “The experimenter, therefore, taking the altitude of the sun and of the rainbow above the horizon, will find that the final altitude at which the rainbow can appear above the horizon is 42°, and this is the maximum elevation of the rainbow.… And the rainbow reaches this maximum elevation when the sun is on the horizon, namely at sunrise and sunset.”
Grosseteste’s theory of the rainbow was improved by Bacon, who realized that the phenomenon was due to the action of individual raindrops, though he erred in rejecting refraction as part of the process. He was also mistaken in his explanation of the colors of the rainbow, which he thought were illusory. He concluded that “the bow appears only in raindrops from which there is reflection to the eye; because there is merely the appearance of colours arising from the imagination and deception of the vision.… A reflection comes from every drop at the same time, while the eye is in one position, because of the equality of the angles of incidence and reflection.”
Although Bacon’s theory of the rainbow was incorrect, his observations and method, following and extending those of Grosseteste, paved the way for those who would eventually arrive at the true explanation. This was finally achieved by Newton in his Optiks, where he explained the rainbow as being due to a combination of reflection, refraction, and dispersion, or the division of sunlight into its component colors, a spectrum extending from red to violet.
Bacon’s tendency toward the occult is evidenced in a number of his statements, for example, where writes that “it has been proved by certain experiments” that life can be greatly extended by “secret experiences.” One of his recommendations for achieving an exceptionally long life involves eating the specially prepared flesh of flying dragons, which he says also “inspires the intellect,” or so he was told “without deceit or doubt from men of proved trustworthiness.” Writings such as this gave Bacon the posthumous reputation of being a magician and diviner who had learned his black arts from Satan.
But this should not obscure the fact that he and Grosseteste were among the founders of the experimental method that gave rise to modern science. Bacon’s account of his method in the Opus maius is an expression of his plea, addressed to Pope Clement IV, for the study of science, one that would be answered by those who followed the path that he and Grosseteste had opened.
The crucial advance made by Grosseteste and his followers beyond their immediate predecessors was explained succinctly by Crombie: “The strategic act by which Grosseteste and his thirteenth- and fourteenth-century successors created modern experimental science was to unite the experimental habit of the practical arts with the rationalization of twelfth-century philosophy.” Although Grosseteste himself did not always adhere to the practice of verifying his theories by experimentation, most of his followers did. As Crombie noted:
In the next generation such natural philosophers as Roger Bacon and Petrus Peregrinus and, later, Theodoric of Freiburg were to use this principle as the basis of some really thorough and elegant pieces of experimental research.
Hence reasoning does not attest these matters, but experiments on a large scale made with instruments and by various necessary means are required. Therefore no discussion can give an adequate explanation in these matters, for the whole subject is dependent on experiment. For this reason I do not think that in this matter I have grasped the whole truth, because I have not yet made all the experiments that are necessary.… Therefore it does devolve on me to give at this time an attestation impossible for me, but to treat the subject in the form of a plea for the study of science.