ure which is of a different outline; nor, even though the surface were in both cases plane, can it appear to me, at the same moment, half an inch square, and many feet square. All this must be done, however, as often as we open our eyes, if there be truly any perception of visible figure coexisting with the mere suggestions of touch. The visible figure of the sphere, on which I fix my gaze, is said to be a plane of two dimensions inseparable from colour, and this inseparable colour must yet be combined with the sphere, which I perceive distinctly to be convex. According to the common theory, therefore, it is at once, to my perception, convex and plane; and, if the sphere be a large one, it is perceived, at the same moment, to be a sphere of many feet in diameter, and a plane circular surface of the diameter of a quarter of an inch. The assertion of so strange a combination of incongruities would, indeed, require some powerful arguments to justify it; yet is has been asserted, not merely without positive evidence, as if not standing in need of any proof, but in absolute opposition to our consciousness; and the only arguments which we can ever imagine to be urged for it, are, as we have seen, of no weight, or would tend as much to prove the original visual perception of tangible figures, as of the figure that is termed vis ible. Is it not at least more probable, therefore, that though, like the particles of odour when they act upon our nostrils, the rays of light affect a portion of the retina, so as to produce on it an image, which, if the eye were separated from its orbit, and its coats dissected, might be a distinct visible figure to the eye of another observer; this figure of the portion of the retina affected, enters as little into the simple original sensation of sight, as the figure of the portion of the olfactory nervous expanse, when it is affected, enters into the sensations of smell?—and that, when the simple affection of sight is blended with the ideas of suggestion, in what are termed the acquired perceptions of vision,-as, for example, in the perception of a sphere,-it is colour only which is blended with the large convexity, and not a small coloured plane ?which small coloured plane being necessarily limited in extent and form, so as never to be larger than the retina itself, cannot blend with various forms and magnitudes, and which, if it could even be supposed to constitute a part of the convexity of a sphere perceived by us, still could not diffuse its own limited and inseparable colour over the whole magnitude of the sphere. I have stated to you my own opinion with respect to visible figure, an opinion, which to myself, I confess, appears almost certain, or, at least, far more probable than the opinion generally entertained, that has no evidence in our consciousness at any one moment of vision to support it. But on subjects of this kind, which are in themselves so very subtile, and, therefore, so liable to error, I must beg you, at all times, and especially when the opposite sentiment has the authority of general belief, to consider any opinion, which I may submit to you, as offered more to your reflection, than for your passive adoption of it. If I wish you,-reverently, indeed, but still freely,-to weigh the evidence of doctrines of philosophy, which are sanctioned even by the greatest names of every age, I must wish you still more, because it will be still more your duty, to weigh well the evidence of opinions that come to you, with no other authority than that of one very fallible individual. In looking back on the senses which we have been considering, what a boundless field do we seem already to have been endeavouring to traverse? and how admirable would the mind have been, even though it had been capable of no other office than that of representing, in the union of all its sensations, as in a living mirror of the universe, the splendid conceptions of the great Being who formed it; or, rather of creating a-new in itself, that very universe which it represents and admires? Such is the power of the senses ;—of "senses, that inherit earth and heavens, Man makes the matchless image, man admires."* * Young's Night Thoughts, VI. v. 420–427. 429–430. and 435–436. 459 LECTURE XXX. HISTORY OF OPINIONS REGARDING PERCEPTION. GENTLEMEN, in my last Lecture, I brought to a conclusion my remarks on Vision, with an inquiry into the justness of the universal belief, that, in the perception of objects by this sense, there are two modifications of extension, a visible as well as a tangible figure; the one originally and immediately perceived by the eye, the other suggested by former experience. I stated, at considerable length, some arguments which induce me to believe, in opposition to the universal doctrine,—that, in what are termed the acquired perceptions of sight, there is not this union of two separate figures of different dimensions, which cannot be combined with each other, more than the mathematical conceptions of a square and a circle can be combined in the conception of one simple figure; that the original sensations of colour, though, like the sensations of smell or taste, and every other species of sensation, arising from affections of definite portions of nervous substance, do not involve the perception of this definite outline, more than mere fragrance or sweetness, but that the colour is perceived by us as figured, only in consequence of being blended by intimate associations with the feelings commonly ascribed to touch. Philosophers, indeed, have admitted, or, at least, must admit, that we have no consciousness of that which they yet suppose to be constantly tak ing place, and that the only figure which does truly seem to us, in vision, to be combined with colour, is that which they term tangible, that, for example, we cannot look at a coloured sphere, of four feet diameter, without perceiving a coloured figure, which is that of a sphere four feet in diameter, and not a plain circular surface of the diameter of half an inch; yet, though we have no consciousness of perceiving any such small coloured circle, and have no reason to believe that such a perception takes place, they still contend, without any evidence whatever, that we see at every moment what we do not remember to have ever seen. After our very full discussion of the general phenomena of perception, as common to all our senses, and as peculiarly modified in the different tribes of our sensations,-I might now quit a subject, to which its primary interest as the origin of our knowledge, has led me to pay, perhaps, a disproportionate attention. But besides the theories, to the consideration of which our general inquiry has incidentally led us, there are some hypothetical opinions on the subject, of which it is necessary that you should know at least the outline,-not because they throw any real light on the phenomena of perception, but because, extravagantly hypothetical as they are, they are yet the opinions of philosophers, whose eminence, in other respects, renders indispensable some slight knowledge even of their very errors. In reviewing these hypotheses, it will be necessary to call your attention to that doctrine of causation, which I before illustrated at great length, and which I trust, therefore, I may safely take for granted that you have not forgotten. In sensation, I consider the feeling of the mind to be the simple effect of the presence of the object; or, at least, of some change, which the presence of the object produces in the sensorial organ. The object has the power of affecting the mind; the mind is susceptible of being affected by the object,—that is to say, when the organ, in consequence of the presence of the external object, exists in a certain state, the affection of the mind immediately follows. If the object were absent, in any particular case, the mind would not exist in the state which constitutes the sensation produced by it; and, if the susceptibility of the mind had been different, the object might have existed, as now, without any subsequent sensation. In all this series of mere changes, or affections, in consequence of certain other preceding changes, or affections, though a part of the series be material, and another part mental, there is truly, as I have repeatedly remarked to you, no more mystery than in any other series of changes, in which the series is not in matter and mind successively, but exclusively in one or the other. There is a change of state of one substance, in consequence of a change of some sort in another substance; and this mere sequence of change after change is all which we know in either case. The same Almighty Being, who formed the various substances to which we give the name of matter, formed also the substance to which we give the name of mind; and the qualities with which he endowed them, for those gracious ends which he intended them to answer, are mere susceptibilities of change, by which, in certain circumstances, they begin immediately to exist in different states. The weight of a body is its tendency to other bodies, varying according to the masses and distances;—in this instance, the quality may be said to be strictly material. The greenness or redness ascribed to certain rays of light, are words expressive merely of changes that arise in the mind when these rays are present on the retina; in this case, the quality, though ascribed to the material rays as antecedent, involves the consideration of a certain change of state in the mind which they affect. But the greenness or redness, though involving the consideration both of mind affected, and matter affecting, is not less conceivable by us as a quality of matter than the weight, which also involves the consideration of two substances, affecting and affected, though both go under the name of matter alone. All the sequences of phenomena are mysterious, or none are so. It is wonderful, that the presence of a loadstone should cause a piece of iron to approach it; and that the presence of the moon, in different parts of the heavens, should be continually altering the relative tendencies of all the particles of our earth. In like manner, it is, indeed, wonderful, that a state of our bodily organs should be followed by a change of state of the mind, or a state of our mind by a change of state of our bodily organs; but it is not more wonderful, than that matter should act on distant matter, or that one affection of the mind, should be followed by another af fection of the mind, since all which we know in either case, when matter acts upon matter, or when it acts upon mind, is, that a certain change of one substance has followed a certain change of another substance,-a change which, in all circumstances exactly similar, it is expected by us to follow again. We have experience of this sequence of changes alike in both cases; and, but for experience, we could not, in either case, have predicted it. This view of causation, however, as not more unintelligible in the reciprocal sequences of events in matter and mind than in |