[641] Opus majus, Bridges, vol. i. p. 106.
[642] Commonly called “mathematica.”
[643] Opus majus (Bridges, i. p. 253). Bacon goes into this matter elaborately.
[644] Cf. S. Vogl, Die Physik Roger Bacos (Erlangen, 1906). Gives Bacon’s sources.
[645] Opus minus, pp. 367-371.
[646] Opus majus, pars v. dist. iii. (Bridges, ii. p. 159 sqq.).
[647] A contemporary of Bacon named Witelo composed a Perspectiva about 1270, following an Arab source; and a few years later a Dominican, Theodoric of Freiburg, was devoted to optics, and wrote on light, colour, and the rainbow. Baeumker, “Witelo, ein Philosoph und Naturforscher des XIII. Jahrh.” (Beiträge, etc., Münster, 1908); Krebs, “Meister Dietrich, sein Leben, etc.” (Baeumker’s Beiträge, 1906).
[648] With Bacon, experientia does not always mean observation; and may mean either experience or experiment.
[649] See Charles, Roger Bacon, pp. 17-18.
[650] Ante, pp. 313-315. Duns Scotus puts clearly the double aspect of logic, which Albertus Magnus approached: “It should be understood that logic is to be considered in two ways. First, in so far as it is docens (instructs, holds its own school): and from its own necessary and proper principles proceeds to necessary conclusions, and is therefore a science. Secondly, in so far as we use it, by applying it to those matters in which it is used: and then it is not a science” (Super universalia Porphyrii, Quaestrio i., Duns Scotus, Opera, t. i. p. 51).