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Theodoric of Freiberg (c. 1250 – c. 1310, also known as 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 is reckoned to be the first to give a correct explanation of the rainbow, in his work De iride et radialibus impressionibus (On the Rainbow and the impressions created by irradiance). In the beginning of the 14th century, he experimented with spherical flasks filled with water to simulate water droplets. His theory was that the lightbeams were refracted when entering the atmospheric droplets, then reflected inside the droplets and finally refracted again when leaving them.

He provided a model for both the primary rainbow (caused by one reflection and two refractions) and the secondary rainbow (caused by two reflections and two refractions), thus explaining why we sometimes see a double rainbow.

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.

• Wallace, W. A. The Scientific Methodology of Theodoric of Freiberg. A Case Study of the Relationship Between Science and Philosophy. Studia Friburgensia, N.S. 26. Fribourg: The University Press, 1959.
• Crombie, A. C. Robert Grosseteste and the Origins of Experimental Science. Oxford: The Clarendon Press, 1953.

• Entry in the Stanford Encyclopedia of Philosophy