Hence they who deny all causative activity, and reduce causality to a particular case of sequence, further conclude, logically enough, that we can know nothing of 'natures,' 'substances,' 'essences,' and 'things in themselves,' but only phenomena. Logic should further lead them to deny all potential being and all permanent existence, and to take up with the Heraclitean flux.

Of God and His Creatures: 3.69