The most important portion of the work was that dealing with nervous action and the mechanism of the senses. Here he stated the principle, previously recognized but not stated as clearly, that the kind of sensation following stimulation of a sensory nerve does not depend on the mode of stimulation but upon the nature of the sense organ. Thus light, pressure, or mechanical stimulation acting on the retina and optic nerve invariably produces luminous impressions. This he termed the law of specific energies of the sense.