Power vs. Substance
Kant conceives of powers (Kraft, vis) as irreducible relations of a substance to its accidents. He uses this position to criticize the Wolfian one (and German rationalism more broadly):
Wolff assumes one basic power and says: the soul itself is a basic power which represents the universe. It is already false when one says: the soul is a basic power. This arises because the soul is falsely defined, as the Ontology teaches. Power is not what contains in itself the ground of the actual representation, but rather the relation
of the substance to the accident, insofar as the ground of the actual representations is contained in it. Power is thus not a separate principle, but rather a relation (Metaphysik Herder ML_1, 28:261 (1762-64)
One worry I have about this is that it isn’t clear to me that there is a real philosophical dispute here (as say Wuerth argues), but rather just a terminological dispute. What does it really mean to say a power is a relation of substance to accident?
Power and Action
It’s actually somewhat unclear to me what Kant’s position comes to, or what the difference between power (Kraft) and action (Handeln) comes to, given the similar way that Kant defines an act. Kant construes the notion of an act (Handlung) as closely connected with the concepts of activity (Tätigkeit), substance, and force (Kraft) (A204/B250; R5289-90 18:144 (1776-78?); R5650 18:298-302 (1785-88)). Kant’s critical conception of causality is also closely linked to these notions, for an act (Handlung) “already signifies the relation of the subject of causality to the effect” (A205/B250). What is an “act”? Kant provides the clearest description in metaphysics lectures from the critical period:
Action [Handeln] and effect [Wirkung] can only be ascribed to substance. Action is the determination of the power/force [Kraft] of a substance as a cause of a certain accident [accidentis]. Causality [Causalitas] is the characteristic of a substance insofar as it is considered as the cause of an accident [accidentis] (Metaphysik Pölitz 28:564-5 (1790/1)).
Perhaps one way to think about this is as a power being the relation of a substance to some set of possible accidents while an act is a relation of a substance to an actual accident? But that can’t quite be right if an act determines a force or power to be the cause of an accident.
Power or Force?
There is another issue – is it even right to translate “Kraft” as “power” here? Certainly the connection with “vis” suggests that it is. But we would now distinguish between a potentiality and a force. A force is actual, it is a reality with an intensive magnitude. A potentiality is not. Perhaps Kant sees these as tied because he considers force as the actualization of potentiality of a substance to bring about change. All activity involves the exertion of force, and thus the realization of a potentiality. Kant’s critical period amodalism also makes this tricky. How should one understand ‘potentiality’ if there are only actualities—i.e. existing beings?