There is a Cambridge University graduate in England who had parts of his brain damaged by a viral infection. Now he cannot recall any experience from the past beyond the last five minutes. He lives continually in the present with the constant feeling that he has just woken for the first time.
He constantly insists that he is dead. He reasons that although he experiences the present, he has no past, and therefore has no real consciousness. That which is without consciousness, he claims, is dead, or at most like an animal.
Do we have the right to tell him that he is human, just because we have the tendency to perceive him as such?
Face this issue! Don't avoid it! Who are you? Are you human?
If we say he is still human, but that his humanity is diminished only by the degree that he lacks all the normal human faculties, then a tree must also be partially human, inasmuch as it shares some faculties with humans.