Quotes from book
Opus Majus

The Opus Majus is the most important work of Roger Bacon. It was written in Medieval Latin, at the request of Pope Clement IV, to explain the work that Bacon had undertaken. The 840-page treatise ranges over all aspects of natural science, from grammar and logic to mathematics, physics, and philosophy. Bacon sent his work to the Pope in 1267, accompanied by a letter of dedication which was found by F. A. Gasquet in the Vatican Library and published in 1897. It was followed later the same year by a smaller second work, his Opus Minus, which was intended as an abstract or summary of the longer work, followed shortly by a third work, Opus Tertium, as a preliminary introduction to the other two.


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„For the things of this world cannot be made known without a knowledge of mathematics.“

—  Roger Bacon, book Opus Majus

Cited in: Opus majus: A translation by Robert Belle Burke. Vol 1 (1962). p. 128
Opus Majus, c. 1267
Context: For the things of this world cannot be made known without a knowledge of mathematics. For this is an assured fact in regard to celestial things, since two important sciences of mathematics treat of them, namely theoretical astrology and practical astrology. The first … gives us definite information as to the number of the heavens and of the stars, whose size can be comprehended by means of instruments, and the shapes of all and their magnitudes and distances from the earth, and the thicknesses and number, and greatness and smallness, … It likewise treats of the size and shape of the habitable earth … All this information is secured by means of instruments suitable for these purposes, and by tables and by canons.. For everything works through innate forces shown by lines, angles and figures.

Roger Bacon photo
Roger Bacon photo
Roger Bacon photo
Roger Bacon photo

„I use the example of the rainbow and of the phenomena connected with it, of which sort are the circle around the sun and the stars, likewise the rod lying at the side of the sun or of a star which appears to the eye in a straight line… called the rod by Seneca, and the circle is called the corona, which often has the colors of the rainbow. But neither Aristotle nor Avicenna, in their Natural Histories, has given us knowledge of things of this sort, nor has Seneca, who composed a special book on them. But Experimental Science makes certain of them. [The experimenter] considers rowers and he finds the same colors in the falling drops dripping from the raised oars when the solar rays penetrate drops of this sort. It is the same with waters falling from the wheels of a mill; and when a man sees the drops of dew in summer of a morning lying on the grass in the meadow or the field, he will see the colors. And in the same way when it rains, if he stands in a shady place and if the rays beyond it pass through dripping moisture, then the colors will appear in the shadow nearby; and very frequently of a night colors appear around the wax candle. Moreover, if a man in summer, when he rises from sleep and while his eyes are yet only partly opened, looks suddenly toward an aperture through which a ray of the sun enters, he will see colors. And if, while seated beyond the sun, he extend his hat before his eyes, he will see colors; and in the same way if he closes his eye, the same thing happens under the shade of the eyebrow; and again, the same phenomenon occurs through a glass vessel filled with water, placed in the rays of the sun. Or similarly if any one holding water in his mouth sprinkles it vigorously into the rays and stands to the side of the rays; and if rays in the proper position pass through an oil lamp hanging in the air, so that the light falls on the surface of the oil, colors will be produced. And so in an infinite number of ways, as well natural as artificial, colors of this sort appear, as the careful experimenter is able to discover.“

—  Roger Bacon, book Opus Majus

6th part Experimental Science, Ch.2 Tr. Richard McKeon, Selections from Medieval Philosophers Vol.2 Roger Bacon to William of Ockham
Opus Majus, c. 1267

Roger Bacon photo
Roger Bacon photo

„Everything in nature completes its action through its own force and species alone… as, for example, fire by its own force dries and consumes and does many things. Therefore vision must perform the act of seeing by its own force. But the act of seeing is the perception of a visible object at a distance, and therefore vision perceives what is visible by its own force multiplied to the object. Moreover, the species of the things of world are not fitted by nature to effect the complete act of vision at once, because of its nobleness. Hence these must be aided by the species of the eye, which travels in the locality of the visual pyramid, and changes the medium and ennobles it, and renders it analogous to vision, and so prepares the passage of the species itself of the visible object… Concerning the multiplication of this species, moreover, we are to understand that it lies in the same place as the species of the thing seen, between the sight and the thing seen, and takes place along the pyramid whose vertex is in the eye and base in the thing seen. And as the species of an object in the same medium travels in a straight path and is refracted in different ways when it meets a medium of another transparency, and is reflected when it meets the obstacles of a dense body; so is it also true of the species of vision that it travels altogether along the path of the species itself of the visible object.“

—  Roger Bacon, book Opus Majus

Bacon, like Grosseteste, asserts that both the active extramitted species of vision from the eye, and the intramitted species of light from object seen, were necessary for sight.
v. i. vii. 4, ed. Briggs as quoted in A.C. Crombie, Robert Grossetest and the Origins of Experimental Science 1100-1700 (1953)
Opus Majus, c. 1267

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„If in other sciences we should arrive at certainty without doubt and truth without error, it behooves us to place the foundations of knowledge in mathematics…“

—  Roger Bacon, book Opus Majus

Bk. 1, ch. 4. Translated by Robert B. Burke, in: Edward Grant (1974) Source Book in Medieval Science. Harvard University Press. p. 93
Opus Majus, c. 1267

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