great influence of some diseases on that power; but it will more properly fall to be considered under Memory itself; and the intellectual debility of age being intimately connected with decay of that faculty, may also be reserved for future consideration.

Before proceeding to the application of these principles to our notions of the qualities of bodies, it may be proper to remark their accordance with the phenomena of attention. The various sensations which we have at the same time do not all equally affect us; our attention is more particularly directed to one of them; nay, that one may even entirely withdraw our attention from all the others. Or if the attention be strongly directed towards some intellectual object, we may even be unconscious for a time of all our sensations together. Thus, if the mind be engrossed with study, a clock may strike at our side without our being sensible of having heard it. On the other hand, every one must have observed that our sensations appear more vivid when our attention is directed to them; we can easily distinguish a well-known voice in the midst of a large chorus, and we can discover slight shades of difference in tastes, which, without attention, would have seemed perfectly similar. Whence, then, is this power? How is it that we can render our sensations more vivid, when it is evident that we cannot increase the force of the impressions? Will it admit of

any other explanation than this, that it arises from the conception which is included in the process of sensation? We can, when we please, form a clearer conception of the sensation; but when the mind is entirely engrossed with the conception of something else, it cannot at the same time be directed to the sensation. In the case we have instanced of the clock striking unnoticed by our side, there can be little doubt that the sound produces a sensation as usual; but the mind being quite engrossed with the contemplation of something else, does not form a conception of that sensation, or at least forms none sufficiently lively to be remembered. Attention, indeed, appears in no case to be any thing farther than the joint operation of conception and judgment, conception to give a lively representation of the subject attended to, judgment to remark the relations of its several parts to one another, and to draw more clearly the line of distinction between it and other subjects. When we give attention to a moral advice, what do we else than form a lively and distinct conception of the duty which is urged on us? When we pay attention to a narrative, what do we else than form clear conceptions of the facts narrated, and of the relations of its different parts? In like manner, when we give attention to a sensation, what do we else than form a more distinct conception of it?

SECTION II.

We have already observed that it is impossible to conceive the same simple, indivisible, immaterial mind to exist at the same time in two different and distinct states. It may incline partly to one state, partly to another, and it is this which constitutes a complex state of mind. Different emotions may co-exist with one another, or with various remembrances, reasonings, conceptions, &c., but they do not co-exist distinctly. In proportion to the number of ingredients in a complex state, each simple ingredient is less discernible; and in proportion as one is more distinct the others are more obscure. Such is the rapidity of which thought is capable, that the mind, by instantaneous transitions, can give the appearance of co-existence to ideas which are in reality successive. But such distinct co-existence is apparent only, and we shall be the less inclined to doubt this if we think of the rapidity which jugglers give even to their bodily movements, so as seemingly to annihilate the time occupied in their sleight of hand tricks. It seems almost intuitively evident that the same simple, indivisible, immaterial substance cannot exist, at one and the same time, in two distinctly different states.

Yet, obvious as, I think, this must appear to every unbiassed mind, it has been strangely overlooked. Had this principle been admitted and attended to, I do not see how the common theory of sensation could have stood its ground for a single hour. In vision a plane of colours is presented to the eye, and every colour produces a distinct sensation How can these sensations co-exist in the mind without being blended? If sensation belong to the mind, every separate colour will cause a different state of mind, and a hundred colours presented to the eye at once, must, if they be distinguished at all, induce a hundred states of mind quite distinct from one another. This is in direct contradiction to the principle we have just laid down. According to it, the different sensations would be blended into one complex state of mind, and thus all the colours would be mingled together as completely as if they had been mixed with the brush of the painter. Instead of the splendour of sunset, the sky streaked with scarlet and gold,

the variegated landscape spread out in an almost endless succession of verdant lawns, and lofty woods, and cultivated fields, and peaceful streams, and distant mountains, instead of all those enchanting scenes that furnish matter for the beautiful descriptions of the poet and the noble imitations of the painter, we should have but one uniform sensation of dirty grey, to which all the glories of the descending sun could only add a

livid shade of red. Such, if the principle which we have stated be true, appears the unavoidable consequence of the common theory of sensation : that principle does operate, at least to a certain extent; those, then, who would deny its universal operation, must show some reason why it should operate in one case and not in another.

No such insuperable difficulty attends the explanation of vision according to that view of sensation which was exhibited in the preceding section of this chapter. According to it, one part of the sentient substance of the retina has the sensation of one colour, another part of another colour; the picture, in short, is felt as it is formed, - an extended plane, with all the different lines, and shades, and colours clearly distinguished. The mind then passing in rapid review over the objects of this picture, or rather, I should say, over these sensations in the retina, becomes acquainted with them individually, until by the instantaneousness of its operations it seems to have but one conception of the whole, accompanied with judgments of the various distances and figures of the objects.

It is already evident that I consider extension to be an object of sight as well as of touch, but that shall be more fully explained presently. In the mean time, let us inquire whether the doctrine which has been advanced will facilitate the explanation of this notable idea of extension.