Luigi Giussani (The Religious Sense 40-44) says that human beings are composed of two realities. The first reality he is the measurable and the second is immeasurable. Zombies are a picture of humanity, bereft of the this second reality.
The material world has the qualities of height and depth and weight and temperature–these are all measurable. To measure is to compare the whole to one of its parts. A can a human body can be broken down into pounds of flesh, quarts of blood, and feet of intestines. Giussani points out that by their very nature material things are can be broken down into parts. This divisibility is closely related to mutability. All material things are subject to change. The severed limb, even if undisturbed by ranging scavengers, will be greatly altered three weeks later. This holds true even if the gutter contained a lower jawbone full of teeth, although the time would be considerably longer for alterations to be noticed.
As human beings, we are aware of the measurable and the mutable–it is part of our identity. But also part of our identity are realities that are indivisible, immeasurable and immutable. Giussani identifies idea, judgment and decision as aspects of the human identity that inherently have these qualities.
Idea: We have an idea in our head of something we call “goodness.” When one of our fellow survivors is being attack from three directions and she will likely succumb, it is good that one of the party goes back to dispatch the zombie attacking her from behind.
Judgement: My declaration, “This is a crowbar” will still be true a billion years after the last human has been bitten by the undead.
Decision: The act of deciding that I like a specific person establishes forever the definition of the relationship.
These things do not change on their own, like the severed limb necessarily does. The ideas, judgments and decisions endure. The decision may be wrong, I may discover the person I liked has chosen to eat the whole can of beans rather than share with me and now I no longer like them, but this is a new decision. Both these decisions are indivisible and unchangeable in themselves.
The point of all of this is to recognize that both the measurable and the immeasurable aspects are part of the experience of our “I”. And we should not reduce our experience to one or the other of these two realities. The zombie is a thought experiment where we make the human soul reducible to material reality.