Kant’s argument that an act out of duty can not be in conflict with itself or with any other will acting out of duty derives from the concept he puts forth of the internal principle. A will cannot conflict itself if it determines itself a priori. By determining its morals before the benefit of experience, it determines itself simply that it exists as it is. Intuitively, anything pure cannot conflict with itself just as the idea of good cannot conflict with itself and be somehow partly bad (437).
Thus by simply being, without any other influence determining it, the will is an end in itself (437). A will acting out of duty, or in other words on its own internal principles, can not conflict with another will simply because it does not depend on the other will. In order to conflict, something must first interact. And if two wills are acting in accordance with duty, then they each recognize each other as an end in itself, and therefore do not interact on the level of morality (438).
Just as a self-sufficient village with no roads leading to or from can not conflict with another village simply because it needs not and cannot interact, a self-sufficient will, and therefore determined with no external influence, can also not conflict with anther will acting out of duty.
Though if something is not self-sufficient, it requires another object to fulfill its ends. As with the village, if it needs to conquer a neighboring village’s farmland in order to feed itself, conflict arises. Similarly, should a will not be determined a priori, but instead based on external circumstances, then a will must use another will to fulfill its needs, and therefore would conflict with the autonomy of the second will.