A follow-up to the last post:
We can see major problems with viewing human beings as "constructed" when we get into the details of who counts as a valuable person and who doesn't. On the constructionist view, human organisms are valuable for certain functional characteristics (they must "look human," be viable, etc. -- they must be "fully constructed" to a certain degree).
But the criteria offered by defenders of abortion and embryo-destructive research always seem too arbitrary to justify radically different treatment of different classes of humans. Richard Stith points out that in the case of constructed objects, such as a car, everyone has their own idea of when the car-in-construction officially counts as a car. Do we really want to take the same approach to valuing human beings? Maybe we aren't "fully human" until we reach maturity at about age 18 or 21, so children and teenagers don't merit full respect and protection.
Constructionist criteria is arbitrary, totally undermines our intuitive notion of human equality (construction is a matter of degree, after all), and can exclude obvious examples of valuable human persons. It seems clear that human beings are not like constructed objects at all. We are living organisms who maintain our identities at each stage of development even as we gain and lose abilities. We have our dignity and our right to life by virtue of what we are, not by virtue of arbitrary, degreed characteristics we may acquire.