In his excellent blog, Justin Taylor quotes from J. P. Moreland on the issue of whether someone can be human but not a person, a distinction used by pro-abortion and pro-euthanasia advocates to justify taking human lives.
In debates about the beginning of life some will concede the scientific reality that at human conception the embryo is a “human being,” but then deny that this is a “person.” The assumption is often that personhood is something that comes in degrees and requires not just certain capacities but actual manifestation of certain properties associated with personhood.
Standardized tests often include questions of the form “A is to B as X is to _____.” This exercise helps to identify relationships of symmetry and can prove illuminating. J. P. Moreland, in a lecture at Southeastern Baptist Theological Seminary on “Naturalism and the Crisis of the Soul,” gives a helpful illustration on this issue:
Being a person is to being human as being a color is to being red. Something can be colored without being red or blue, but something could not be red without being colored.
Someone can be a person without being a human (an angel), but someone can not be a human without being a person.
Being red is one way of being a color, and being a human is one way of being a person.
When we think of it this way, we can make sense out of how there could be colored things that are not red things or persons without being humans (God, angels, Martians). But it does not make any sense to say there could be a human non-person any more than it would make sense to say there could be a red object that was not colored.
For an advanced level discussion on personhood, Thomistic substance dualism, and bioethics, I’d highly recommend Moreland and Rae’s Body & Soul: Human Nature & the Crisis in Ethics (IVP, 2000).