C.S. Lewis's sermon "Transposition" contains in it an excellent layman's summary of what is now known as property/substance dualism.
The fundamental principle of "Transposition" is that "higher mediums" can't be transposed/represented by "lower mediums" without the loss of something. The higher can always be explained by the lower--emotions can always be identified with physical states--because lower mediums have fewer means of representation available to them. Drawing has two dimensions while reality three. A true understanding of the lower is only available from the vantage point of the higher. Drawings are only seen for what they are when one has seen the real object.
The most interesting part of Lewis's sermon, though, is when he departs from his Neoplatonic heritage and rejects introspection. "The attempt to discover by introspective analysis our own spiritual condition is to me a horrible thing which reveals, at best, not the secrets of God's spirit and ours, but their transpositions in intellect, emotion, and imagination, and which at worst may be the quickest road to presumption or despair." Augustine, who so extensively analyzed the structure of his own psyche in a book called Confessions, is somewhere in Africa rolling in his grave.
Matthew Lee Anderson is the Founder and Lead Writer of Mere Orthodoxy. He is the author of Earthen Vessels: Why Our Bodies Matter to our Faith and The End of Our Exploring: A Book about Questioning and the Confidence of Faith. Follow him on Twitter or on Facebook.