Some bioethicists, such as Baruch Brody, believe that full humanness begins when the brain starts functioning, which can first be detected by the electroencephalogram (EEG) at about 40 to 43 days after conception.
Although Brody has moral problems with abortion on demand prior to brain functioning, this is not because he believes the unborn is fully human. Brody maintains that in order to decide when something is fully human, “we must first see… what properties are such that their loss would mean the going out of existence (the death) of a human being.” 
He concludes that since at brain death a human being goes out of existence (at least in this mortal realm), the presence of a functioning human brain is the property which makes one fully human. Hence, it would only follow that the start of brain functioning is the beginning of full humanness.
The fundamental difficulty with this argument is that…
That is to say, an entity's irreversible absence of brain waves after the brain waves have come into existence indicates that the entity no longer has the natural, inherent capacity to function as a human being, since our current technology is incapable of “reactivating” the brain. However, the unborn entity who has yet to reach the stage in (his or) her development at which brain waves can be detected, unlike the brain dead individual, possesses the inherent capacity to have brain waves. She is like a patient with a temporarily flat EEG.
Brody responds to this criticism by presenting the following science-fiction case:
But consider the entity whose brain has died. Is he not like the fetus? Both have the potential for developing into an entity with a functioning brain (we shall call this a weak potential) but we can conclude, it seems to me, that an entity can go out of existence even if it retains a weak potential for having a functioning brain, and that, analogously, the fetus is not a human being just because it has this weak potential.
I do not believe that this response succeeds.
First, unlike the potential of the corpse's dead brain to be liquified and recast as a new brain, the unborn's potency to develop is within itself (intrinsic). “As in the case of other organisms,”" philosopher A. Chadwick Ray points out, the unborn's development “admittedly requires nourishment from outside and an appropriate environment (consider parasites), but still, the fetus has within itself the power to appropriate nourishment and grow.”
On the other hand, the potential of Brody's corpse is utterly extrinsic. That is, “it can be acted upon from the outside and brought to life, but without immediate surgery its life will not be restored, and it will simply rot.”
Second, the unborn has “interests of itself, in a straightforward, non-projective way, that go beyond the interests of its component parts—cells, tissues, etc.,” just as I as a living organism have interests that go beyond the interests of my component parts—ears, nose, teeth, etc. On the other hand, the corpse “has no interests beyond those of its parts. The component cells may have an interest in continuing to live, but the corpse itself has none.”
For example, “there would be no loss in the corpse's organs, all being donated to different patients (imagine donating every living cell if you prefer), whereas in a living fetus's being chopped up for spare parts its own interests would be sacrificed.” 
In summary, “the growth of the fetus is in its own interest and is the realization of its intrinsic potential, in which realization its identity is preserved.” However, “the implanting of a new brain into a brainless corpse would constitute the genesis of a new organism with its own new telos and interests where there were none.”
Therefore, since the prebrain-functioning unborn entity has a natural inherent capacity for brain functioning while the corpse does not, they do not have the same kind of weak potential that Brody claims they have.
Author: Francis J. Beckwith, adapted from a series in Christian Research Journal, Spring 1991. Provided with permission by Summit Ministries and the author.
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