2. A native of Tralles in Lydia, and one of the founders of the medical sect of the Methodici (Galen, Introd. 100.4, vol. xiv. p. 684.). He lived at Rome in the reign of the emperor Nero, A. D. 54-68 (Plin. Nat. 29.5), to whom he addressed one of his works (Galen, De De Meth. Med. 1.2, vol. x. pp. 7, 8); and here he died and was buried, and his tomb was to be seen in Pliny's time on the Via Appia, with the arrogant title of , it having been his constant boast during his life that medicine surpassed all other arts, and that he surpassed all other physicians. (Galen, ibid. p. 11.) He was the son of a weaver, and followed the same employment himself during his youth. (Galen, ibid. p. 10.) This, however, he soon gave up, and, though he had had a very imperfect general education, he embraced the medical profession, by which he acquired for a time a great reputation, and amassed a large fortune. He adopted the principles of the Methodici (Dict. of Ant. s. v. Methodici), but modified and developed them so much that he attributed to himself the invention of them, and indeed is always considered as one of the founders of the sect. In fact he appears to have endeavoured on all occasions to exalt himself at the expense of his predecessors (Pliny, l.c.); lavishing upon the ancients the most insulting epithets, asserting that none of them had contributed any thing to the advance of medical science (Galen, ibid. p. 8), and boasting that he could himself teach the art of healing in six months. (pp. 4, 5.) He is frequently mentioned by Galen, but always in terms of contempt and ridicule ( De Meth. Med. i. l, &c., vol. x. p. 4, &c.; Adv. Julian. c. l, &c. vol. xviii. pt. i. p. 247, &c.); probably this was well deserved, as it agrees with what is said of him by Pliny (l.c.), but still the gross personal abuse in which Galen indulges goes beyond all bounds, and is quite unworthy of so great a man. An account of the opinions of Thessalus may be found in Le Clerc's Hist. de la Méd., Haller's Biblioth. Medic. Pract. vol. i., and Sprengel's Hist. de la Méd. vol. ii. Perhaps it need only be noticed here that he was the inventor of what he called μετασύγκπισις (rendered by Caelius Aurelianus, De Morb. Acut. 2.38, p. 173, " recorporatio"), a method which still forms our principal and most essential corporeal means in the treatment of insanity. His object was, in obstinate chronic cases, where other remedies failed, or were not indicated, to effect a thorough commotion in the fundamental constitution of the organism (σύγκπισις). To this end he commenced by the application, both internally and externally, of strong vegetable remedies, to the use of which, together with the strictest regimen and emetics administered at intervals, a period of three days was devoted. This treatment was preparatory to a system of fasting, and concluded with a course of restoratives. (See Feuchtersleben's Medical Psychology, chap. 2, p. 38.) He wrote several medical works, of which only the titles and a few sentences remain : 1. Κάνων, "Canon." (Gal. De Meth. Med. 4.4, vol. x. p. 268; De Simplic. Medicam. Temper. ac Facult. 5.25, vol. xi. p. 783.) 2. Περὶ τῶν Κοινοτήτων, "De Communitatibus" (id. De Meth. Med. 1.2, vol. x. p. 7.) 3. Περὶ Χειρουργίας, " De Chirurgia," (id. ibid. 4.4. p. 250). 4. Συγκριτικά (id. ibid. p. 7), probably the work called " Comparatio" by Caelius Aurelianus. (De Morb. Acut. 3.17, p. 247.) 5. A Letter to the Emperor Nero. (Gal. l.c. p. 7.) 6. " De Regulis," which might have been considered to be the same work as the " Canon " mentioned above, but that Caelius Aurelianus quotes it as the book "De Regulis, quas Graeci Diactas vocant" (De Morb. Acut. 3.17, p. 247); it is therefore possibly the same work which this author elsewhere quotes as " Liber Diaeteticus" (ibid. i. l. p. 11) or as " Liber Regularis" (De Morb. Chron. praef. p. 268), or perhaps the whole work may have been called " Canon," of which the second book was the " Liber Diaeteticus." (id. ibid. 2.8. p. 387.) The reputation of Thessalus does not seem to have been very lasting, as, with the exception of Galen and Pliny, Caelius Aurelianus and Soranus ( De Arte Obstetr. pp. 128, 210, 212), both of whom belonged to the sect of the Methodici, are perhaps the only ancient authors who mention him.

Fabricius mentions (Bibl. Gr. vol. xiii. p. 436, ed. vet.) a third physician of the name of Thessalus, and refers to Justin, 12.13; but the true reading in that passage is " Medius Thessalus," not Medicus ; and Medius, not Thessalus, is the proper name. [MEDIUS, § 2,]