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nitional rank, but it is to the fact that the eye affords an immediate presentation of surface extension, that its fundamental importance as a source of objective knowledge is due. The apprehension of colour necessarily involves that of space in two dimensions. It is undoubtedly true that originally the single eye, if it remained in a fixed position, could have apprehended but a very limited quantity of surface, that its perception of shape would have been extremely vague, and that it could have afforded no information at all as regards distance; but nevertheless the sensation. of colour necessarily implies some perception of extension. The point will be made clearer when we come to treat of the development of senseperception; here, however, we would note that the means by which our visual perceptions of shape and distance are elaborated, and our apprehension of surface enlarged, are changes in the position and form of the eye made known to us by muscular sensations. The movement of the axis of the eye round the object viewed, the convergence of the two eyes varying with its distance, the selfadjusting process by which the optical lens is flattened or rendered more convex so as to focus the object upon the retina, are accompanied by faint feelings of tension which play an important part in giving precision to our spatial cognitions. In mature life the "local" sensibility of the retina is very fine. Close to the centre of the yellow spot irritations as near together as '004 mm. are felt as distinct; but the discriminative power diminishes

as we pass towards the circumference. The size of the retinal image, of course, decreases with the distance of the object, still this extreme delicacy of the retina to the local character of the irritation enables the eye to become a very perfect instrument for the accurate appreciation of extension.

As a direct source of pleasure or pain visual sensations rank probably lower than those of any other faculty, though indirectly they may contribute much to our happiness. Bright lights and hues are pleasing, and harmonious combinations have an agreeable effect. An intense glare of light is painful, but the feeling is organic rather than visual. Prolonged confinement in the dark produces an intense desire for light and great joy on first restoration to liberty, but the pleasure soon fades. The contemplation of the beauties of nature and art affords rich and refined delight, but here the effect is of an intellectual and emotional character, and not an immediate function of the sense.

In our last chapter we remarked on the inverse ratio subsisting between the perceptional and the pleasurable or painful capacity of the senses. Glancing back at them now, when they have been separately passed under review, and their chief features described in detail, the truth of that observation will be realized. If we divide our tactual consciousness into the two great groups, the organic sensations, including the feelings of temperature on the one side, and the muscular feelings and sensations of touch proper on the other, and proceed to arrange them first according

to emotional, and then in regard to cognitional rank, we shall find that the two schemes will assume virtually an inverse order. Viewed as direct sources of pleasure and pain, starting from the highest they seem to stand thus: organic sensation, taste, smell, hearing, muscular and tactual states, and sight. But marshalled as instruments of objective knowledge the order is reversed: sight, tactual and muscular sensations, hearing, smell, taste, and lowest, the organic feelings. This classification regards only the immediate or direct emotional and cognitional properties of the consciousness of each sense, and the intrinsic difficulties of all such comparison would probably cause diversity of view about the former scheme; still, estimated from this limited standpoint, it seems to us approximately correct.

Indirectly, indeed, sight is a much more important source of pleasure and pain than the sense of smell, and the knowledge of the universe acquired by hearing far exceeds that gathered from the actual experience of all our other senses combined; but in both cases we have merely appropriation of the results attained by the other faculties, and extension of these results by means of association and inference. Viewed purely as a state of feeling, a sensation of colour or sound can afford much less pleasure or pain than an agreeable odour, or a nauseous stench. Similarly, the sensations of hearing are more precise, more finely discriminable, and more vividly revived in imagination, not only than those of taste and smell, but even than our tactual and muscular consciousness. Yet, inasmuch

as they give us immediately no assurance of the reality, or of the extension of the material world, they must be ranked cognitionally higher than taste or smell, but lower than the combined muscular and tactual sense. Touch, indeed, since it reveals the mechanical properties of the world, has claims to stand even before sight as an instrument of objective cognition, and it is certainly more necessary; still, the immense range of the latter faculty, its perfect presentation of the geometrical relations of the universe, and the delicacy of its other cognitive aspects have led us to place it at the head of the list.15 We need not attempt any further justification of the arrangement adopted, as the reader, by returning on our treatment of the senses separately, may ascertain the various considerations which have led to our conclusion.

Readings. On classification of the senses, cf. St. Thomas, Sum. i. q. 78. a. 3; De Anima, II. 11. 22—24, et III. 1. 1; De Sensu et Sensato, 1. I. On the various senses, cf. De Anima, II. ll. 13-24, De Sensu et ̧ Sensato, Lib. I. Of modern works on the special senses, cf. Wyld, Physics and Philosophy of the Senses, Pt. III.; Ladd, op. cit. Pt. I. c. v. and Pt. II. cc. iii. iv. The Five Senses of Man, by Bernstein, is a good popular treatise in many respects, but the author frequently confuses in a very crude manner the physical and the psychological processes. A good account of the senses containing many valuable remarks on the views of Aristotle and the schoolmen in the light of modern physiological science, will be found in P. F. Salis Sewis' work, Della Conoscenza Sensitiva.

15 Balmez, Fundamental Philosophy, Bk. II. cc. x. xi. maintains the inferiority of touch to sight and hearing from a cognitional point of view. He does not, however, distinguish sufficiently in this question between the direct or immediate efficacy of a sense and that which is merely mediate. In range and representative power the more refined senses vastly surpass touch, but to a very large extent their wealth is built upon the capital supplied by the more fundamental faculty.

CHAPTER VI.

PERCEPTION OF THE MATERIAL WORLD:

CRITICAL

SKETCH OF THE LEADING THEORIES OF

EXTERNAL PERCEPTION.

How do we perceive the External Material World, and what are our grounds for believing in its real existence? This is the problem which has most harassed Philosophy since the days of Descartes. The two questions, the Nature of external perception and the Validity of our belief in a material universe, are most intimately bound up with each other. The worth of every theory of cognition must be estimated by the sufficiency of the account which it gives of the reality that is known. Accordingly, though only the question of the process of apprehension is of a strictly psychological character, while the validity of the act belongs to Applied Logic, or Metaphysics, we shall find it very advantageous in the interests of our own science to trespass here a little on the domain of another volume of the present series. This impossibility of separating the problems of the genesis and the truth of knowledge shows again the futility of all attempts at isolating Phenomenal Psychology from Rational Psychology and Philosophy proper.

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