nityaṃ kāryānumeyā ca śaktiḥ kimanuyujyate / tadbhāvabhāvitāmātraṃ pramāṇaṃ tatra gamyate
Why should any question be raised about capacity, which is always inferable from [its] effects? The only proof in this matter is that [the effect] exists only when that [capacity] exists.