  As to the debate about the unicity of substantial form, Giles’ position evolves during time. If we leave aside the Errores philosophorum, since the authenticity of this work has been contested with serious arguments (Bruni 1935, Koch 1944, Donati 1990b, Luna 1990), we can notice that Giles changes his position from the Contra gradus et pluralitatem formarum (between the end of 1277 and the beginning of 1278), where he, against Henry of Ghent denies plurality of forms for every compound (Wilson 2014), to later works, where he takes a more cautious stance in particular concerning man. The principle of individuation is identified, as in Aquinas, with “materia signata quantitate”, that is, matter designated by its dimensions.  According to Giles, who criticizes  also Richard of Mediavilla on this point, matter is pure potentiality and therefore cannot be distinguished into different kinds. For this reason, he cannot accept Aquinas’ doctrine that the incorruptibility of celestial bodies derives from the peculiar nature of their matter (Donati 1986). In Giles’ opinion celestial bodies are not incorruptible because their matter is different from the matter of sublunar bodies, but rather because their quantitas materiae cannot change its determinate dimensions. This is but one application of Giles’ famous doctrine of “indeterminate dimensions”. Modifying Averroes’ doctrine in this respect, Giles argues that a portion of matter, in order to be able to receive a form, needs to possess already a sort of quantity. Such quantity, however, should not be identified with the determinate dimensions a body possesses, but is rather a quantitas which remains the same during processes such as rarefaction and condensation. Giles’ notion of quantitas materiae, which is not only generically extension or three-dimensionality, but seems to represent an unchangeable given “amount” of matter pertaining to a body, has been considered comparable, some differences notwithstanding, to the modern notion of mass (Donati 1988, Donati-Trifogli 2016).