Thomas asks ( ST III, 60, 5) whether the sensible thing of the sacraments is a “determinate” something. Do we have to use specific things, or may we substitute at will?

His answer is that sacramental elements are determinate, and his reasoning has to do with the nature of sacramental causation. Sensible things in general are “endowed with natural powers” and if two things have the same power, the choice between them is indifferent. If two medicines have the same effect, it doesn’t matter which we use. But the sacraments don’t possess the power to sanctify by their nature. Water can cleanse, but it cannot be a means of spiritual cleansing on its own. Sacraments have power “only in virtue of the Divine institution.” God must thus “determine the sensible things to be employed in the sacraments.”

Though of course Thomas applies it differently, this reasoning doesn’t seem at odds with the Protestant insistence that sacraments must be instituted by Christ.