According to psychiatrist Viktor Frankl, there must be a healthy core to a human being. If there were not a healthy core to a person, say, with mental illness, then he or she could lose his or her humanity, becoming non-human by virtue of the severity of his or her illness. Thus, the most mentally ill man or woman is still a human being, a person.
Similarly, there must a healthy core to the most vicious human beings. If there were not, they could become non-human by virtue of their evil acts. Thus, even the most wicked human beings still remain humans.
Likewise, neither old age nor terminal illness, neither Alzheimer’s disease nor dementia, makes a human being become non-human. A human being retains his or her humanity up to his or her last breath, to the moment of death.
No human being, then, can change himself or herself from one nature (what something is, its essence) to another. The healthy core of a human being is his or her spirit and that is the human essence, making him or her a person. Therefore, once a human person, always a human person.