( Or Thierry of Saxony).

A philosopher and physician of the Middle Ages, and a member of the Order of Saint Dominic. We cannot with any degree of certainty identify him with Frater Theodoricus, who in 1283, was named prior of the convent of Würtzburg, but there is abundant evidence that at the Chapter General held at Strasburg (1293) he was made superior general of the province of Germany, holding this post until 1296. In 1297 we find him at Paris, teaching the "Sentences" of Peter Lombard ; in 1303, at Coblenz; and in 1304, at Toulouse, taking part in the Chapter General of his order. In 1310, the Chapter General of Plaisance appointed him Vicarius Povinciæ Teutonicæ , while awaiting the nomination of a new provincial. Nothing is known of his after life.

Thierry was a very active writer. A list of the works of Dominican authors, compiled in 1330, ascribes to him thirty-one different treatises, twenty-one of which are still in existence, on the most diverse subjects of theology, metaphysics, and cosmology. But the one which especially redounds to the glory of Thierry is that composed in 1304 "De Fride" for the Chapter General of Toulouse, at the request of Améric de Plaisance, superior-general of the order. Therein with wonderful clearness Thierry describes the different reflections and refractions of every ray which forms either the first or the second rainbow. This experiment was made with a spherical drop of water. Furthermore, with the help of spherical glass vases filled with water, he verified experimentally the phenomena which he planned. This work, which made its author a precursor of Descartes, is a model of the art of logically combining experiments.

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