It is time to inquire who was Marcus Græcus. He has been fancifully identified with many of the Marci of history and fable, and M. Dutens discovered him in the second century A.D. The views of M. Dutens must be noticed here, because they have been unwarily adopted by some good writers. There exists a Latin translation of a lost Arabic treatise on medicine, _De Simplicibus_, supposed by some to have been written by Masawyah of Damascus in the eleventh century,[174] while others, with M. Dutens, assign it to Yahya ibn Masawyah, who attended the Caliph Mamoun on his deathbed,[175] 833 A.D. The question before us is, does the _De Simplicibus_ (whatever its date) contain any reference to Marcus? When mentioning the use of syrup of cyclamen, Masawyah quotes the opinions of other physicians: “The son of Serapion said (so and so) ... and a Greek (physician) says (so and so)” (_dixit filius Serapionis ... et dicit Græcus_).[176] On the last two words, _dicit Græcus_, M. Dutens builds his theory that the Greek physician was no other than Marcus: “Ce qui paroît fort probable, est que (Marcus Græcus) devoit vivre avant le médicin arabe, Mesué, qui a paru au commencement du neuvième siècle, puisque celui-ci le cite.[177] By this mode of reasoning, which is generally called “begging the question,” Marcus Græcus may be identified at will with any Greek who ever lived. M. Dutens continues: “Fabricius croit que (Marcus Græcus) est le même dont Galen parle dans un endroit de ses ouvrages, au quel cas il serait du temps requis pour appuyer mon sentiment.” It would be strange indeed to find mention of a Latin writer or book in a _Bibliotheca Græca_, and I have failed to verify M. Dutens’ reference. In the editions of Fabricius’ work which I have consulted he expresses no such belief, nor does he allude to Marcus Græcus. In the list Fabricius gives of ancient physicians there are several who bear the name of Marcus, but no Marcus Græcus. The last of these Marci happens to be simply called “Marcus,” and of him Fabricius says: “This Marcus, who is mentioned by Galen in his book on compounding medicines, may possibly have been one of the foregoing” (_Marcus, simpliciter, Galeno in compositionibus medicamentorum_ κατὰ τόπους, _l._ iv. _c._ 7, _quem credibile fuisse unum ex illis prioribus_).[178] The _Liber Ignium_ was written from first to last in the period of literary forgeries and pseudographs, which produced the “Book of Hermes,” “The Domestic Chemistry of Moses,” the alchemical works of Plato and of Aristotle and of the Emperor Justinian, and so on; and we may reasonably conclude that Marcus Græcus is as unreal as the imaginary Greek original of the tract which bears his name. Had the last editor of the _Liber Ignium_, who added the saltpetre receipts, any knowledge of an explosive?