The author of the Summa Halensis claims that the human body is maximally composite and argues for this using a proof strategy that intends to deduce the body’s composition from the human soul’s immateriality. This study examines that claim and argument, which is given both in a shorter and a longer form. The core of the article consists in a careful reconstruction of both forms, along with an enquiry into its Jewish Neoplatonic sources (first and foremost the Fons vitae) and (...) its appearance in zoological commentaries contemporary to the Summa written by Peter of Spain and Albert the Great. It emerges that the argument brings into play various features of the Summist’s hylomorphic theory, especially a pluralism about substantial forms. (shrink)