may enter into the estimate. Thus, in judging the height of an unfamiliar object, such as a rock, or a mound of earth afar off, we are led to form an idea of the length of space intervening by the number and apparent magnitude of known objects between us and the point in question, by the apparent size of other known figures, such as those of men or animals situated in its vicinity, and by the clearness or mistiness of the outlines of the object and of its neighbours.

The education of the sense of sight proceeds concomitantly with that of the faculty of tactual and motor sensations. Mutually aiding each other their progress is very rapid. The advantages gained by touch through the consciousness of double-contact are now largely increased by the addition of a power which can apprehend in an instant the entire contour of the body, and the situation of the various agents acting upon it. The length of the sweep of the arm or leg are known not merely in the dim terms of subjective motor feelings, but through the fine visual perceptions of space. The wide range of the eye, and those other numerous excellences which have been detailed in describing this sense, confer upon its acts the power of arousing with marvellous facility and speed the representation of associated tactual and muscular sensations. By this singularly perfect appropriation of the acquisitions of touch, vision is enabled to inform us in an easy, rapid, and admirable manner of a multitude of the tangible properties of things which we could never, or but by an incredible amount of labour, ascertain

through actual contact. At the same time, the control of the organ of sight is secured by the ciliary muscles; and while we watch the movement of the arm, the muscular sensations of the eye reveal the quantity of change in its own direction, the degree of convergence of the optic axes, and the increase or decrease in the convexity of the crystalline lens. In this way by the mutual cooperation of the two faculties our knowledge of the most important attributes of matter is elaborated.12

AUDITORY PERCEPTION.-The ear gives us originally no knowledge of the spatial relations of the external world, nor even of the nature of the objective cause of the sensations of sound. Of the acquired perceptions of this faculty the most remarkable are the sense of the direction of a sounding body, and the sense of its distance. Both are due to association, and neither of them reach in man a very high degree of perfection. If while our eyes are closed a noise is produced near us by the concussion of two objects, such as keys, we shall find it almost impossible to localize the sound, especially when the experiment is performed above our head or near our feet. In mature life we estimate the distance of a familiar sound by means of its intensity. If it is of a rare character, such as that of thunder or of the explosion of gunpowder, we

12 Vision, unlike touch, taste, and smell, does not seem to be capable of much advance in range or refinement beyond what it normally reaches. The skill with which the Indian can follow a trail and the sailor recognize an object at sea seem among the remarkable effects of special education of this sense. Unlike the other faculties, sight is normally developed almost up to its full maximum efficiency.

most

The discrimination of

feel completely at a loss. direction is dependent on the difference in the effects produced in the two ears, and also on the variation in the character or intensity of the sound brought about by moving the head. An object on the right side makes a stronger impression on the right than on the left ear, and the sound is intensified by bringing the head or body to that side, or by setting the ear in a more direct line with the sonorous object. Hares and other animals endowed with large movable ears far surpass man in this respect. Careful cultivation may extend considerably the power of distinguishing faint sounds, and we find certain uncivilized tribes, as well as some species of the lower animals, in which this sense has been developed to a surprising degree as a means of ascertaining the advent of their foes or their prey. Its imperfection as an informant regarding space is partially redeemed by the fineness of its appreciation of time lengths, and to this quality its value not merely as the musical faculty, but as the instrument of social communication, is largely due.

GUSTATORY AND OLFACTORY PERCEPTION.Neither the sense of taste nor that of smell afforded us originally an immediate perception of external reality. If we make the experiment of tasting a liquid of moderately sweet or sour flavour, which is at the same temperature as the organ, we shall find that even in mature life the resulting sensation is of a vague ill-defined character, and contains little more direct reference to the external world than a

headache, or a general feeling of depression. In experience, however, special tastes have been found to be invariably excited by objects possessing particular tactual and visual qualities, and therefore the three classes of sensation come to be associated so that either may recall the others. By cultivation this faculty can be developed in a very surprising degree, and the expert can detect variations in the flavour of tea, wine, and other articles so faint as to be utterly imperceptible to the ordinary mortal. The first odours which assailed our nostrils probably awoke us up to an indefinite pleasurable or painful feeling, and to nothing more. But after a time we grew to associate certain smells with particular objects known by touch and sight, and as the higher activities of the mind unfolded themselves we began to apprehend the former as the cause of the latter. To the circumstance that this sense is stimulated by effluvia of distant bodies, much of its superiority to taste, both in point of refinement and of cognitional importance, is due. As revealing future gustatory experiences, and giving timely warning of poisonous or unwholesome food, olfactory perception becomes an instrument of considerable value. This faculty, like that of taste, is susceptible of a high degree of cultivation, and in the absences of some of the other senses it has reached a remarkable degree of acuteness.

The account we have just given of the gradual growth of perception obviates various difficulties urged against the doctrine of Natural Realism. Mr.

Bain, for instance, objects against Hamilton that the terms "external," "independent," and "reality"

are not simple and ultimate notions, but complex and derived," and consequently that "it is inadmissible to regard any proposition involving them as an ultimate fact of consciousness." 13 Undoubtedly these terms in ordinary language imply a variety of elements which it would be absurd to assert are all given in the "primitive unanalyzable dictum of consciousness." Accordingly, to maintain that the first sensation of pressure or sight revealed to the infant a material world as external, independent, and real, in the full significance of these words, would be as unjustifiable as to hold that the first glance at a triangle or circle presents to us all its geometrical properties. Starting from impressions of sight and touch which vaguely present to us extended reality other than our perceiving mind, our present well-defined knowledge of our own sentient organism, and of objects external to it, becomes gradually elaborated. The continuous existence of these realities when unperceived, which especially establishes their independence of the Ego, is guaranteed by memory, reflexion, and inference, and not by direct intuition. Finally, through the same means we learn to distinguish between the illusions of the imagination and the genuine deliverances of the external senses, and so come to comprehend the full meaning of reality.

The objection that we cannot have an immediate knowledge of an "external reality," that "it is

13 Mental Science, p. 120.