For a short explanation have a look at wholeness. "The term PSYCHO-PHYSICAL is used both here and throughout my works to indicate the impossibility of separating 'physical' and 'mental' operations in our conceptions of the working of the human organism. [...] In my opinion the two must be considered entirely interdependent, and even more closely knit than is implied by such a phrase." If you think Alexander's definition sounds complex, it certainly fits into his time.