[T]he problem is that a unique abstract characterization of man’s cognitive functioning does not exist. … The fact that it is not possible to uniquely determine cognitive structures and processes poses a clear limitation on our ability to understand the nature of human intelligence. I once thought it could mean unique identification of the structures and processes underlying cognitive behavior. Since that is not possible, I propose that we take ‘understanding the nature of human intelligence’ to mean possession of a theory that will enable us to improve human intelligence.1