Kant- experiences

“But experience teaches us what exists and how it exists, but never that it must necessarily exist so and not otherwise (Pg. 35).” During this part of the book, Kant is trying to justify his belief that nature can neither be a priori or a posteriori. I am unsure of my feelings on if nature is in fact one or the other or neither. However, this reason for why nature is not a posteriori is not sufficient enough for me. Kant says that experiences teach us how things exist, right? SO then how can he say that does not mean it necessarily exist so? Obviously, it does because if it has been experienced it must exist. Perhaps the outcome may change of nature, and the experience may be altered at another time, but if one experience had occurred one way than it is in fact so a possibility to occur, maybe even again. Kant makes no sense with this quote, it is a major contradiction. If experience teaches us, than how can he say what we experienced doesn’t necessarily exist? It must exist if it happened, and we learned from it???....