Epistemology (truth) and ethics (good) constitute the two main aspects of Kant's philosophy. The former deals with the phenomena of natural causality, while the later deals with the noumena of free will. Phenomena and noumena are, in fact, necessity and freedom, and knowledge and morality in Kant's philosophy, and constantly engage in confrontation. While theoretical reason (cognition) is unable to reach the moral realm, practical reason (ethics) can act in the cognitive realm.