Lewis suggests that human beings experience two realities–one linked to the physical world and the other to the spiritual. Luigi Giussani (The Religious Sense 40-44) distinguishes these two realities as well, describing the first as measurable and the second is immeasurable.
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 of Coke can be broken down into millilitres, a human body into pounds and inches. 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. If a student puts the apple on my desk on the last day of school, I will find the gift greatly altered by the following September. This holds true even if the gift was a diamond, although the time would be considerably longer for the alterations to be noticed.
As human beings, we are aware of the measurable and the mutable–it is part of our identity. We are material, or animal as Lewis says.
But we are aware of something else that is just as essentially part of us as the material elements–an immutable element. Giussani identifies idea, judgment and decision as aspects of the human individual that are unchanging, indivisible and unmeasurable. He offers an example of each:
Idea: We have an idea in our head of something we call “goodness.” When I was a child, I thought my mother good. Even after all these years, I use the same criteria to determine that my mother is still good–this idea is unchanging.
Judgement: My declaration, “This is a piece of paper” will still be true in a billion years.
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 diamond or the apple necessarily do. The ideas, judgments and decisions endure. The decision may be wrong, I may discover the person I liked had betrayed me and now I no longer like them, but this is a new decision. Both are indivisible and unchangeable in itself.
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 important conclusion one can draw from all this is that the animal (body) and the spiritual (soul) are not reducible to each other.