the concept of cause cannot arise in this [empiricist] way at all, but must either be grounded in the understanding completely a priori or else be entirely surrendered as a mere fantasy of the brain. For this concept always requires that something A be of such a kind that something else B follows from it necessarily and in accordance with an absolutely universal rule. Appearances may well offer cases from which a rule is possible in accordance with which something usually happens, but never a rule in accordance with which the succession is necessary; thus to the synthesis of cause and effect there belongs a dignity that can never be expressed empirically, namely that the effect does not merely come along with the cause, but is posited through it and follows from it. [A 91/B 123-4]