C.G. Jung Letters, Vol. 1: 1906-1950
It does not surprise me that psychology debouches into philosophy, for the thinking that underlies philosophy is after all a psychic activity which, as such, is the proper study of psychology.
I always think of psychology as encompassing the whole of the psyche, and that includes philosophy and theology and many other things besides.
For underlying all philosophies and all religions are the facts of the human soul, which may ultimately be the arbiters of truth and error. ~Carl Jung, CW 8, Para 535