What is the theme in What Dreams May Come: A Novel by Richard Matheson?

Chris discovers the power of his mind when he chooses to ignore the pain of the accident during his time in Borderland. At first this seems just to be an interesting occurrence. Then, when Chris meets Albert in Summerland, he learns that a house can be constructed by concept, that travel can be instantaneous if one just visualizes it. More and more, Chris begins to understand just how powerful the mind is. During his descent into the lower realm, Chris can palpably feel the oppression and negativity there and it manifests itself in his posture and his inability to maintain his light. Albert's ability is stronger, and this example is yet another proof to Chris of the power of the mind.