(3) The law of interest, the uniform principles governing the influence of the feelings upon the will in its earlier stages, these are an enigma which only an exhaustive knowledge of the physiology of the nervous system, of heredity, and possibly of many other as yet unsuspected factors, could enable us to solve. Leibniz applied his doctrine of petites perceptions to its solution, and certainly unconscious elements, whether inherited or stored up from personal experience, have much to do with our actual volitions, and lie at the very bottom of character and temperament; but as yet there is no science, nor even prospect of a science, of these things.
http://www.newadvent.org/cathen/15624a.htm
attention/interest will need to be explored at a later date.
