The Six Books of Proclus on the Theology of Plato, Volume 2

For if it is in the effect, it either gives completion to it, or is in a certain respect indigent of it in order to its existence, and thus it will be more imperfect than the thing caused. For being in the effect, it is rather a concause than a cause, and is either a part of that which is generated, or an instrument of the maker. For that which is a part in the thing generated, is more imperfect than the whole. The cause also which is in the effect, is an instrument of generation to the maker, being unable to define of itself the measures of production. Every cause, therefore, which is properly so denominated, if it is more perfect than that which proceeds from it, imparts to its effect the measure of generation, and is exempt from instruments and elements, and in short, from every thing which is called a concause.