Perception is a vital part of how the three versions of a person or an object interact with each other. The Spiritual Realm filters through the Cognitive Realm to get to the Physical Realm. While the Spiritual Realm contains the ideal version of something, the Cognitive filter makes sure the Physical version matches their perception of what should be. One of the ways this manifests is in healing. For example, until he swears his Fourth Ideal, Kaladin feels he deserves his slave brands, so they don’t heal. Lopen, on the other hand, never viewed himself as disabled, so his arm grew back. The application of this can extend to changing the body of a transgender person to match their identity or rejoining Siamese twins that were separated.