I believe that Kant defended the "law of cause and effect" by stating this argument:
(P) If we didn't understand or acknowledge the law of cause and effect, we couldn't have any knowledge.
(Q) We have knowledge.
Therefore: (P) we acknowledge the law of cause and effect.
Isn't this line of reasoning a fallacy? P implies Q, Q, : P
You have certainly put your finger on a complex issue. One might say you've got a dragon by the tail. First, I should call your attention to the fact that you've rendered his argument in two logically different ways. The first rendering is actually a valid form of deductive inference, not a fallacy. Philosophers, in their pretentious way, call it a modus tollens. The terms in which you've put it allow for this rendering: 1. If Not-P, then Not-Q. 2. Q. 3. Therefore, P. And, by the way, that first rendering can also be restated in another valid form called a modus ponens: 1. If we have knowledge (Q), then we understand or acknowledge the law of cause and effect (P). 2. We have knowledge (Q). 3. Therefore, we understand or acknowledge the law of cause and effect (P). There's a rather large issue lurking here, too, as to what "understanding" and "acknowledging" mean, how they're similar, how they're different. (See, for example, Stanley Cavell's, "Knowing and...