And so it is not the case that heat is a cause of heat because of some other power that exists in it; rather, it is a cause of heat solely because God has so determined Himself that in the presence of heat-- and not unless it is present--He regularly wills to produce heat. And if He had not so decided, then the very same heat, which would in that case exist without any change, would be heat and yet would not be a cause of heat. Hence, God does nothing through a secondary cause without its being the case that He does it through Himself just as principally and no less so than if He were doing it alone.

gabriel biel, whether the sacraments of the new covenant are effective causes of grace, p.3