In this connection the question arises as to the existence of physiological limits to the exercise of the ordinary senses. In the case of the eye a _minimum visibile_ is generally assumed; and there is special interest in a case of clairvoyance versus cornea-reading, where, if the words were read (as appears most probable) from their reflection upon the cornea of the hypnotiser, the common view as to the _minimum visibile_ is greatly stretched.[79] With regard to the other senses, whose mechanism is less capable of minute dissection, one meets problems of a rather different kind. What are the definitions of smell and touch? Touch is already split up into various factors--tactile, algesic, thermal; and thermal touch is itself a duplicate sense, depending apparently on one set of nerve-terminations adapted to perceive heat, and another set adapted to perceive cold. Taste is similarly split up; and we do not call anything taste which is not definitely referred to the mouth and adjacent regions. Smell is vaguer; and there are cognate sensations (like that of the presence of a cat) which are not referred by their subject to the nose. The study of hyperæsthesia does in this sense prepare the way for what I have termed heteræsthesia; in that it leaves us more cautious in definition as to what the senses are, it accustoms us to the notion that people become aware of things in many ways which they cannot definitely realise. Let us now consider the evidence for heteræsthesia;--for the existence, that is to say, under hypnotic suggestion, of any form of sensibility decidedly different from those with which we are familiar. It would sound more accurate if one could say "demanding some end-organ different from those which we know that we possess." But we know too little of the range of perceptivity of these end-organs in the skin which we are gradually learning to distinguish--of the heat-feeling spots, cold-feeling spots, and the like--to be able to say for what purposes a new organ would be needed. For certain heteræsthetic sensations, indeed, as the perception of a magnetic field, one can hardly assume that any end-organ would be necessary. It is better, therefore, to speak only of modes of sensibility.