Summa Theologica

We must therefore say otherwise, that an efficient cause is twofold, principal and instrumental. The principal cause works by the power of its form, to which form the effect is likened; just as fire by its own heat makes something hot. In this way none but God can cause grace: since grace is nothing else than a participated likeness of the Divine Nature... But the instrumental cause works not by the power of its form, but only by the motion whereby it is moved by the principal agent: so that the effect is not likened to the instrument but to the principal agent

Thomas Aquinas. Summa theological, III, q.62, in First Complete American Edition in Three Volumes. Vol. 2. Trans. Fathers of the English Dominican Province. (New York,NY: Benziger, 1947), #’s. (NEED NUMBERS paragraph or page?)

Thomas explains instrumental causality. This is significant because it helps us to understand how a sacramental may be a cause of divine grace, without threatening divine liberty or ascribing power to the priest that he does not have.

I think that when many contemporary authors wring their hands about sacramental power being seen as magic (think here Rivard, Collins/Power) that they are seeing the effects of neglecting this distinction. In the face of this "medieval accretion" many authors try to re-imagine blessing, and even the concilium members seem to have neglected this when authoring the praenotanda.

feb 11, 2011 - this may also be likened to the use of sacramentum tantum, sacramentum and res, res tantum in order to distinguish between the sign, and the thing signified properly.