This article first provides information on the life and work of the physician Urso of Salerno, who was active at the end of the 12th century. It then examines the relations between Urso's work and the Latin translations of Aristotle's libri naturales. It studies the concept of yle or prime matter in the De commixtionibus elementorum libellus, the only work in which Urso discusses yle. Urso explains that yle is created from nothing by the opifex, and he describes its characteristics at length. Once natural processes have begun, yle disappears, and the reciprocal transformations of the elements occur instantaneously, without the intermediary stage of yle.
