Opus Majus of Roger Bacon, Vol. II (Burke English trans.)

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world. Hearing causes us to believe because we trust our teachers, but we cannot test what we learn except through vision. Moreover, if we should consider taste, touch, and smell, we are assuming knowledge belonging to beasts. For brutes are occupied with things pertaining to taste and touch, and they exercise their sense of smell because of taste and touch. However, these are of little value, few in number, and are concerns common to us and to brutes; therefore, they do not rise to the rank of human wisdom.

But sciences are formed because of necessity, utility, and difficulty, since art has to do with what is difficult and good, as Aristotle says in the second book of the Ethics. For if what is sought is easy, there is no need to form a science. Likewise, although a matter may be difficult, if it is not useful, no science is developed concerning it because the labor would be foolish and vain. Unless a subject were very useful and possessed many excellent truths, it would not require the formation of a separate science; it would suffice to treat it in some particular book or chapter alongside other matters in general science. But concerning vision alone is a separate science formed among philosophers—namely, optics—and not concerning any other sense. Therefore, there must be a special utility in our knowledge through vision that is not found in the other senses.

What I have touched upon in general, I wish to show in particular by disclosing the basic principles of this very beautiful science. It is possible that some other science may be more useful, but no other science has so much sweetness and beauty of utility. Therefore, it is the flower of all philosophy, and through it—and not without it—can the other sciences be known. We must note, moreover, that Aristotle first treated this science, regarding which he says in the second book of the Physics that its subject is placed under another head. He also mentions it in his book on Sense and the Sensible, and he has proven Democritus in error because he did not name the refractions and reflections of vision with reference to the optic and concave visual nerves. This book has been translated into Latin. After him, Alhazen treats the subject more fully in a book which is extant. Alkindi has also arranged some data more fully, as have other authors of books on vision and mirrors.