Leland said that it is easier to collect the leaves of the Sibyl than the titles of the works written by Roger Bacon; and though the labour has been somewhat lightened by the publications of Brewer and Charles, referred to below, it is no easy matter even now to form an accurate idea of his actual productions. An enormous number of MSS. are known to exist in British and French libraries, and probably not all have yet been discovered. Many are transcripts of works or portions of works already published and, therefore, require no notice.
The works hitherto printed (neglecting reprints) are the following: - (1) Speculum Alchimiae (1541) - translated into English (1597); French, A Poisson (1890); (2) De Mirabili Potestate Artis et Naturae (1542) - English translation (1659); (3) Libellus de Retardandis Senectutis Accidentibus (1590) - translated as the "Cure of Old Age," by Richard Brown (London, 1683); (4) Sanioris Medicinae Magistri D. Rogeri Baconis Anglici de Arte Chymiae Scripta (Frankfort, 1603) - a collection of small tracts containing Excerpta de Libra Avicennae de Anima, Breve Breviarium, Verbum Abbreviatum, Secretum Secretorum, Tractatus Trium Verborum, and Speculum Secretorum; (5) Perspectiva (1614), which is the fifth part of the Opus Majus; (6) Specula Mathematica, which is the fourth part of the same; (7) Opus Majus ad Clementem IV., edited by S. Jebb (1733) and J. H. Bridges (London, 1897); (8) Opera hactenus Inedita, by J. S. Brewer (1859), containing the Opus Tertium, Opus Minus, Compendium Studii Philosophiae and the De Secretis Operibus Naturae; (9) De Morali Philosophia (Dublin, 1860, see below); (10) The Greek Grammar of R. Bacon and a Fragment of his Hebrew Grammar, edited with introduction and notes by E. S. Nolan and S. A. Hirsch (1902); (11) Metaphysica Fratris Rogeri, edited by R. Steele, with a preface (1905); (12) Opera hactenus inedita, by Robert Steele (1905).
How these works stand related to one another can only be determined by internal evidence. The smaller works, chiefly on alchemy, are unimportant, and the dates of their composition cannot be ascertained. It is known that before the Opus Majus Bacon had already written some tracts, among which an unpublished work, Computus Naturalium, on chronology, belongs probably to the year 1263; while, if the dedication of the De Secretis Operibus be authentic, that short treatise must have been composed before 1249.
It is, however, with the Opus Majus that Bacon's real activity begins. It has been called by Whewell at once the Encyclopaedia and the Organum of the 13th century.
Part I. (pp. 1-22), which is sometimes designated De Utililate Scientiarum, treats of the four offendicula, or causes of error. These are, authority, custom, the opinion of the unskilled many, and the concealment of real ignorance with pretence of knowledge. The last error is the most dangerous, and is, in a sense, the cause of all the others. The offendicula have sometimes been looked upon as an anticipation of Francis Bacon's Idola, but the two classifications have little in common. In the summary of this part, contained in the Opus Tertium, Bacon shows very clearly his perception of the unity of science and the necessity of encyclopaedic treatment.
Part II. (pp. 23-43) treats of the relation between philosophy and theology. All true wisdom is contained in the Scriptures, at least implicitly; and the true end of philosophy is to rise from the imperfect knowledge of created things to a knowledge of the Creator. Ancient philosophers, who had not the Scriptures, received direct illumination from God, and only thus can the brilliant results attained by them be accounted for.
Part III. (pp. 44-57) treats of the utility of grammar, and the necessity of a true linguistic science for the adequate comprehension either of the Scriptures or of books on philosophy. The necessity of accurate acquaintance with any foreign language and of obtaining good texts, is a subject Bacon is never weary of descanting upon. A translator should know thoroughly the language he is translating from, the language into which he is translating, and the subject of which the book treats.
Part IV. (pp. 57-255) contains an elaborate treatise on mathematics, "the alphabet of philosophy," maintaining that all the sciences rest ultimately on mathematics, and progress only when their facts can be subsumed under mathematical principles. This fruitful thought he illustrates by showing how geometry is applied to the action of natural bodies, and demonstrating by geometrical figures certain laws of physical forces. He also shows how his method may be used to determine some curious and long-discussed problems, such as the light of the stars, the ebb and flow of the tide, the motion of the balance. He then proceeds to adduce elaborate and sometimes slightly grotesque reasons tending to prove that mathematical knowledge is essential in theology, and closes this section of his work with two comprehensive sketches of geography and astronomy. That on geography is particularly good, and is interesting as having been read by Columbus, who lighted on it in Petrus de Alliaco's Imago Mundi, and was strongly influenced by its reasoning.
Part V. (pp. 256-357) treats of perspective. This was the part of his work on which Bacon most prided himself, and in it, we may add, he seems to owe most to the Arab writers Kindi and Alhazen. The treatise opens with an able sketch of psychology, founded upon, but in some important respects varying from, Aristotle's De Anima. The anatomy of the eye is next described; this is done well and evidently at first hand, though the functions of the parts are not given with complete accuracy. Many other points of physiological optics are touched on, in general erroneously. Bacon then discusses vision in a right line, the laws of reflection and refraction, and the construction of mirrors and lenses. In this part of the work, as in the preceding, his reasoning depends essentially upon his peculiar view of natural agents and their activities. His fundamental physical maxims are matter and force; the latter he calls virtus, species, imago agentis, and by numberless other names. Change, or any natural phenomenon, is produced by the impression of a virtus or species on matter - the result being the thing known. Physical action is, therefore, impression, or transmission of force in lines, and must accordingly be explained geometrically.