 In the Latin West, Avicenna and Averroes were known as the principal adversaries on a much-discussed question of element theory, especially in the fourteenth century. Given that all physical substances (apart from the elements themselves) are mixtures of elements, how do the elements exist in them? (Maier 1952; Grant 1974, §77, Eichner 2005, 139–145). Avicenna’s answer is that the substantial forms of the elements remain unaltered when a compound is formed; only the qualities of the elements are altered and unite to a mean quality (qualitas media), or complexion (complexio). The complexion disposes the matter to receive the substantial form of the compound from the active intellect, the giver of forms (dator formarum) (The Healing: Physics I.10, On generation 6). The problem with this position, as many scholastics saw, is that several bodies are combined in one, which do not form a true mixture. Averroes rejects Avicenna’s theory and argues that the substantial forms of the elements are diminished in the compound (Comm. magnum De caelo III.67). The form of the compound is “composed” of the elementary forms (Comm. magnum Metaph. XII.22). In order not to violate Aristotle’s principle that substantial forms cannot be diminished or augmented (a man is not more man than another), Averroes argued that elementary forms are not substantial forms in the full sense.