Douai, 691 (sec. xvii), wanting chapters 38-52: this MS. has been described by Victor Cousin, _Journal des Savants_ for 1848 (5 articles). Printed in Bacon’s _Opera Inedita_ (Rolls Series), pp. 3-310. Charles has been misled by a passage in the work called ‘_Communia Naturalium_’ into thinking that this latter formed part of the _Opus Tertium_; Charles, _R. Bacon_, pp. 65, 83-4; his description of _Opus Tertium_ is consequently erroneous. The passage is from the Mazarine MS. of the _Communia Naturalium_ (i.e. No. 3576), fol. 85: ‘Quod est improbatum in secunda parte primi operis, deinde in hoc tertio opere explanavi hoc et solvi objectiones.’ These words refer to Bacon’s doctrine that the _intellectus agens_ is not part of the soul, but God and angels. This is insisted on in the _Opus Tertium_, cap. xxiii, and it is not likely that Bacon would do more than refer to it again casually in the course of the same work. The relation of the _Opus Tertium_ to the _Commun. Nat._ is probably as follows: the latter was written or begun first. Bacon repeatedly mentions that he was, while writing his three _Opera_ for the Pope, engaged on a larger work, _Scriptum Principale_, which he did not send to Clement[1349]. Much of this larger work naturally found its way, probably in a summarised form, into the _Opus Tertium_ as we know it, the treatise actually sent to the Pope. _Tractatus de multiplicatione specierum_, or, _De generatione specierum et multiplicatione et corruptione earum_, is inserted by Jebb in the _Opus Majus_, pp. 358-445, between Part v and Part vi. The subject is however discussed in Part iv, which is often quoted or referred to in Part v. In the _De multiplicatione_, &c. (p. 368), are the words: Ut tactum est in communibus naturalium. Again (p. 358): Recolendum est igitur quod in tertia parte hujus operis tactum est, quod essentia, substantia, natura, potestas, potentia, virtus, vis, significant eandem rem, sed differunt sola comparatione. There is nothing about this in the third part of the _Opus Majus_; but it is found in the _Communia Naturalium_. The treatise _De multiplicatione specierum_ was therefore part of a work of which the _Communia Naturalium_ formed the third part. This large work was according to Jebb, the _Opus Minus_; according to Charles, the _Opus Tertium_[1350]; according to Brewer, the encyclopaedic _Compendium Philosophiae._ Brewer is no doubt right; the _De multiplicatione_ was intended as a sub-section of the great treatise on Physics.