Arab philosophy. In the next two sections, I present Thomas's and Scotus's views. I end with a number of conclusions on Scotus's position. 1 The Aristotelian Theory of Mixture The development of the ontology of mixture in Latin philosophy took place in commentaries to Aristotle's De Gen. et corr., in lectures on the second book of Peter Lombard's Sentences that discuss the creation of matter and organisms, in some elucidations of the composition of material substance from De Coelo, Physica, and Meteorologica IV, and in the medical literature on Avicenna's Canon and Galen's doctrine of the humors. While these places sometimes bring about important elaborations and are of great significance for the historical transmission of ideas, the core of the late medieval doctrine of mixts was formulated withrespect to chapter 10 of book I of Aristotle's De Gen. and with respect to Averroes's position. Mixts are sublunary bodies composed of the four primary elements (fire, water, earth, air). They make up the entire material world and are the subjects of natural science. In their ontological representation, the doctrine of hylomorphic composition intervenes at every level of the physical architecture: cats have matter and form, flesh and bones have matter and form, and the four elements themselves have matter and form. The need to describe mixed bodies in terms of metaphysical parts, as 5 Cf. a summary, largely based on Maier, in Wood (2011). John Duns Scotus and the Ontology of Mixture 317 composites of form and matter, gave rise to an enduring medieval debate: the