On of the most radical truths of Christianity is that man is essentially, not simply accidentally, a body-mind-soul union. Mark C. writes that from the moment of conception, God endows the human body with a human soul. I think it is deeper than that. God creates the human soul as he creates the human body becase the human soul and the human body are necessarily and essnetially conjoined. God could not create a human body without a human soul because humanity is by definition a body-soul union (Not that God is incapable of creating just a body or just a soul, but by definition He would be creating an animal or an angel). On either side of this truth are the errors we see today. On the one hand there are those who deny the existence of a soul (or even a mind--as distinct from biochemical or neural impulses) altogether--leading to stuff PETA's "animals are human too" position because humans are just animals. On the other side, and this is where Sen. Kerry seems to be, there is the idea that there is a essential human--a soul that is not necessarily related to the body. The body is not something that makes one who one is, but rather is an insturment to be used for the benefit of the immaterial soul and may be discarded or modified at will--there is no moral content the the use of the body per se. Thus, the mere fact of the existence of a biologically human body does not necessarily imply the existence of an immaterial human soul because body and soul are related only accidentally and not essentially.
This of course, is all erroneous, as Evangelium Vitae, notes. Because a human being is fundamentally a body-soul union, inseparably joined in essence without confusion or admixture, the existence of a live, biologically human body conclusively proves the existence of a human soul, and therefore a human person in and of that body. This is why daemonic possession is so bad and why death is so bad. The separation of body and soul caused by possession or death divorces what was created good as a unity.