By Reductio ad Absurdum, we assume that “only rational agents deserve moral consideration”.
Here we also adapt Kantian’s belief that the existence of ration is a priori and universal.
Then, we may consider the patients of psychological disease, and in persistent vegetative state. According to standard definition of ration, we can say that people in these situations are not rational agents. And for people in PVS, we can even expect that s/he will not become rational again. Therefore, according to our assumption, we can deduce that these patients do not deserve moral consideration. However, for Kantian’s, it seems this result is not acceptable since human should always deserve moral consideration.
Moral, human should be considered as an end