Of Embryos and Brains

While not "not arguing in favor of the brain/nervous system position," Steve says  "[i]f it is argued that a human organism becomes a human being only when it develops a nervous system and a brain, the failure to recognize embryos as human beings does not undercut the ground for believing that infants etc. are human beings."

There is a problem with this logic, I think.  It is true, a person could say "I will not consider a human organism a human being entitled to full worth and dignity until it develops a nervous system and a brain.  Therefore, I conclude that an infant is a human being but a first trimester fetus is not."  But, this opens the door to someone else coming along and saying:  "No, you have it wrong, the human organism does not become a human being until it can engage in rational thought."  And, there are any number of variations.  The point is, you have switched from saying that the human organism has worth because of what it is to saying that a human organism has worth because it happens at the moment to possess certain characteristics, and this does undercut the argument that infants possess full moral worth and dignity.  What do you think Steve?