Theodoric of Freiberg (c. 1250 – c. 1310, also known as Thierry de Fribourg, Thierry of Freburg, Dietrich of Freiberg, Theodoricus Teutonicus de Vrîberg, or simply Meister Dietrich) was a German member of the Dominican order and a theologian and physicist. He was named provincial of the Dominican Order in 1293, Albert the Great's old post.
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While 13th century authors failed to provide an explanation for the rainbow, at the turn of the fourteenth century Theodoric was able to give the first correct geometrical analysis of this phenomenon, which was "probably the most dramatic development of 14th- and 15th-century optics". [1]
Drawing from his two earlier works on light and colour, he wrote De iride et radialibus impressionibus (On the Rainbow and the impressions created by irradiance, c. 1304-1311), relying on geometry, experiment, falsification and other methods. Among other properties he explained in detail:
Using spherical flasks and glass globes filled with water, Freiberg was able to simulate the water droplets during rainfall. Still in its early stages, experimental instrumentation would later expand to be used primarily for making measurements, extending the human senses and creating and isolated environment for the experimenter. During his experimentation with these glass globes, Freiberg was correct in asserting that the colors formed in interaction with the water droplets.
Recently, scientists have found evidence of the experimental instrumentation used by Freiberg. Currently on loan to Universities in Providence, Rhode Island, the instrumentation does, indeed, simulate the droplet of water which the rainbow is reflected through.
One of his contemporaries, Kamal al-Din al-Farisi, offered the same experimentally-established explanation of the rainbow (without any contacts between them) in his Kitab tanqih al-manazir (The Revision of the Optics). Both authors however relied on the Book of Optics by Ibn al-Haytham (Alhacen)/ Alhazen.[2]
Dietrich's theological works tend to be heavily Neoplatonic, while his more secular philosophical works are more Aristotelian. Dietrich disagreed with Thomas Aquinas on certain metaphysical issues, and seems to have written in opposition to particular works by Aquinas.
He had a remarkable influence on the 10 years younger Meister Eckhart, mainly via the treatises De visione beatifica (Of the beatific vision) and De intellectu et intelligibili (Of the intellect and the intelligible), and one of his extraordinary contributions to medieval philosophy was a theory of the soul that equalled the Aristotelian notion of "agent intellect" and the Augustinian notion of "abditum mentis" (i.e. the hiddenness, or hidden place of the soul).
The theory of the agent intellect says that in knowing, the mind is not merely passive, it has to work on producing a conception of its object, a conception which is then received and retained by the passive part of the mind. The hiddenness of the soul, in turn, is the ground of the soul in which God's image is imprinted, a spiritual apex of man's being by which he transcends space and time.