Kants Philosophie der Notwendigkeit

Kant's reflection on necessity is of crucial importance within the radical transformation of metaphysics in the 18th century. On the other hand, it is very difficult to understand his pre-critical works at all and to grasp the idea of a new system of philosophy as a critique of pure reason if one does not comprehend Kant's engagement with the concepts of possibility, reality, and necessity (and thus with the foundations of classical ontology itself) in detail. What does necessity and being necessary mean for Kant?.