This view of nature Bacon considered fundamental, and it lies, indeed, at the root of his whole philosophy. To the short notices of it given in the 4th and 5th parts of the Opus Majus, he subjoined two, or perhaps three, extended accounts of it. We possess at least one of these in the tract De Multiplicatione Specierum, printed as part of the Opus Majus by Jebb (pp. 358-444). We cannot do more than refer to Charles for discussions as to how this theory of nature is connected with the metaphysical problems of force and matter, with the logical doctrine of universals, and in general with Bacon's theory of knowledge.
Part VI. (pp. 445-477) treats of experimental science, domina omnium scientiarum. There are two methods of knowledge: the one by argument, the other by experience. Mere argument is never sufficient; it may decide a question, but gives no satisfaction or certainty to the mind, which can only be convinced by immediate inspection or intuition. Now this is what experience gives. But experience is of two sorts, external and internal; the first is that usually called experiment, but it can give no complete knowledge even of corporeal things, much less of spiritual. On the other hand, in inner experience the mind is illuminated by the divine truth, and of this supernatural enlightenment there are seven grades.
Experimental science, which in the Opus Tertium (p. 46) is distinguished from the speculative sciences and the operative arts in a way that forcibly reminds us of Francis Bacon, is said to have three great prerogatives over all other sciences: - (1) It verifies their conclusions by direct experiment; (2) It discovers truths which they could never reach; (3) It investigates the secrets of nature, and opens to us a knowledge of past and future. As an instance of his method, Bacon gives an investigation into the nature and cause of the rainbow, which is really a very fine specimen of inductive research.
The seventh part of the Opus Majus (De Morali Philosophia), not given in Jebb's edition, is noticed at considerable length in the Opus Tertium (cap. xiv.). Extracts from it are given by Charles (pp. 339-348).
As has been seen, Bacon had no sooner finished this elaborate work than he began to prepare a summary to be sent along with it. Of this summary, or Opus Minus, part has come down and is published in Brewer's Op. Ined. (313-389), from what appears to be the only MS. The work was intended to contain an abstract of the Opus Majus, an account of the principal vices of theology, and treatises on speculative and practical alchemy. At the same time, or immediately after, Bacon began a third work as a preamble to the other two, giving their general scope and aim, but supplementing them in many points. The part of this work, generally called Opus Tertium, is printed by Brewer (pp. 1-310), who considers it to be a complete treatise. Charles, however, has given good grounds for supposing that it is merely a preface, and that the work went on to discuss grammar, logic (which Bacon thought of little service, as reasoning was innate), mathematics, general physics, metaphysics and moral philosophy. He founds his argument mainly on passages in the Communia Naturalium, which indeed prove distinctly that it was sent to Clement, and cannot, therefore, form part of the Compendium, as Brewer seems to think.
It must be confessed, however, that nothing can well be more confusing than the references in Bacon's works, and it seems well-nigh hopeless to attempt a complete arrangement of them until the texts have been collated and carefully printed.
All these large works Bacon appears to have looked on as preliminaries, introductions, leading to a great work which should embrace the principles of all the sciences. This great work, which is perhaps the frequently-referred-to Liber Sex Scientiarum, he began, and a few fragments still indicate its outline. First appears to have come the treatise now called Compendium Studii Philosophiae (Brewer pp. 393-519), containing an account of the causes of error, and then entering at length upon grammar. After that, apparently, logic was to be treated; then, possibly, mathematics and physics; then speculative alchemy and experimental science. It is, however, very difficult, in the present state of our knowledge of the MSS., to hazard even conjectures as to the contents and nature of this last and most comprehensive work.
Bacon's fame in popular estimation has always rested on his mechanical discoveries. Careful research has shown that very little can with accuracy be ascribed to him. He certainly describes a method of constructing a telescope, but not so as to lead one to conclude that he was in possession of that instrument. Burning-glasses were in common use, and spectacles it does not appear he made, although he was probably acquainted with the principle of their construction. His wonderful predictions (in the De Secretis) must be taken cum grano salis; he believed in astrology, in the doctrine of signatures, and in the philosopher's stone, and knew that the circle had been squared. For his work in connexion with gunpowder, the invention of which has been claimed for him on the ground of a passage in his De mirabili potestate artis et naturae, see Gunpowder.
The 13th century, an age peculiarly rich in great men, produced few, if any, who can take higher rank than Roger Bacon. He is in every way worthy to be placed beside Albertus Magnus, Bonaventura, and Thomas Aquinas. These had an infinitely wider renown in their day, but modern criticism has restored the balance in his favour, and is even in danger of erring in the opposite direction. Bacon, it is now said, was not appreciated by his age because he was in advance of it; he is no schoolman, but a modern thinker, whose conceptions of science are more just and clear than are even those of his more celebrated namesake. In this view there is certainly some truth, but it is much exaggerated. As a general rule, no man can be completely dissevered from his national antecedents and surroundings, and Bacon is not an exception. Those who take up such an extreme position regarding his merits have known too little of the state of contemporary science, and have limited their comparison to the works of the scholastic theologians.
We never find in Bacon himself any consciousness of originality; he is rather a keen and systematic thinker, working in a well-beaten track, from which his contemporaries were being drawn by theology and metaphysics.