beryls. His experiments led to an explanation of the reflection/refraction phenomenon of drops of water in clouds that form the rainbow. One of Theodoric's 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. Theology Theodoric'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). Particularly, his theory of transcendentals clarified Eckhart's theory of the negatio negationis. It stressed the negative moment that determines privatio in general even in its privatio privationis formulation. 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. Works Theological works De visione beatifica De corpore Christi mortuo De dotibus corporum gloriosorum De substantiis spiritualibus et corporibus futuræ resurrectionis. Philosophical works De habitibus De ente et essentia De magis et minus De natura contrariorum De cognitione entium separatorum et maxime animarum separatarum De intelligentiis et motoribus cælorum De corporibus cælestibus quoad naturam eorum corporalem De animatione cæli De accidentibus De quiditatibus entium De origine rerum prædicamentalium De mensuris De natura et proprietate continuorum De intellectu et intelligibili. Scientific works De luce et ejus origine De coloribus De iride et radialibus impressionibus De miscibilibus in mixto De elementis corporum naturalium. Modern editions Opera omnia = Corpus Philosophorum Teutonicorum Medii Ævi, Hamburg, Felix Meiner Verlag, vol. 1-4: Burckhard Mojsisch (ed.), Schriften zur