The man who sits down in his study to write or read,--in perfect safety and intent on his work,--continues nevertheless to be involuntarily and inevitably armed with all that alertness to external sights and sounds, and all that sensibility to pain, which protected his lowly ancestors at different stages of even pre-human development. It is much as though he were forced to carry about with him all the external defences which his forefathers have invented for their defence;--to sit at his writing-table clad in chain-mail and a respirator, and grasping an umbrella and a boomerang. Let him learn, if he can, inwardly as well as outwardly, to get rid of all that, to keep at his command only the half of his faculties which for his purpose is worth more than the whole. Dissociation and choice;--dissociation between elements which have always hitherto seemed inextricably knit;--choice between faculties which till now we have had to use all together or not at all;--such is the promise, such is the incipient performance of hypnotic plasticity in its aspect of _inhibitive suggestion_. I come now to the division of hypnotic achievement with which I next proposed to deal, namely, the _dynamogenic_ results of hypnotic suggestion. These I shall arrange in an order resembling that which we try to follow in education:--proceeding from external senses to internal sensory and other central operations; and thence again to attention and will, and so to character which is a kind of resultant of all these. I will begin, then, with what seems the most external and measurable of these different influences--the influence, namely, of suggestion upon man's _perceptive_ faculties;--its power to educate his external organs of sense. This wide subject is almost untouched as yet; and there is no direction in which one could be more confident of interesting results from further experiment. The exposition falls naturally into three parts, as suggestion effects one or other of the three following objects: (1) Restoration of ordinary senses from some deficient condition. (2) Verification of ordinary senses;--hyperæsthesiæ. (3) Development of new senses;--heteræsthesiæ. (1) The first of these three headings seems at first sight to belong to therapeutics rather than to psychology. It is, however, indispensable as a preliminary to the other two heads; since by learning how and to what extent suggestion can repair _defective_ senses we have the best chance of guessing at its _modus operandi_ when it seems to excite the _healthy_ senses to a point beyond their normal powers.[77]