Grosseteste treats bodies as composites of a first or prime matter and a corresponding first form. This form, termed “corporeity,” is the form in virtue of which something is a body.  Grosseteste takes bodies to be more or less metaphysically simple depending on whether, in addition to corporeity, they include other substantial forms. Thus, in On Light he holds the firmament or outermost heavenly sphere to be the most simple body, as it is composed simply of first matter and first form, but in his commentary on the Physics he speaks of the element fire as having in addition to first form the substantial form of fire.