Mere sensation of any kind, reason on it how we will, cannot account for the perception of external objects, which is another and separate fact. We are reduced to admit that it is by a simple primary law of our constitution that the organs of sense (which may with equal propriety be called the organs of perception) convey to us a knowledge of the external world. We touch, and a tangible extended body is made known to us; we open our eyes, and a visible body is before us.
Dr Brown, adopting and refining upon Berkeley's theory of vision, attributes originally nothing more than the mere sensation of colour to the eye, which sensation, by association with that of touch, becomes extended, so to speak, over an external surface, and defined into limited figures. We are not disposed to lay any greater stress than Dr Brown himself upon the image said to be traced upon the retina; but we say that the eye, as well as the touch, immediately informs us of external surface and definite figure.
There is, it is true, a sensation of colour apart from the perception. This may be separated, in our reflection, from all external surface. It is a pleasure which colour gives, and which enters largely into the complex sentiments of beauty. But our notion of colour itself we cannot dissociate from external surface: we cannot think of colour but as something outward. And if it comes to us originally under the condition of external surface, it must also present itself originally under certain forms and figures; for only where the whole field of vision is occupied by one unvaried colour, as when the eye is fixed upon a cloudless sky, could there be the perception of surface without some figure, more or less defined on it.
And why is it, that on a subject of this nature the manifest facts witnessed in the whole animal creation are to be overlooked? If other animals evidently, on the first opening of their eyes, see form, and movement, and the whole world before them; does not this sufficiently intimate the instantaneous knowledge which it is the nature of vision to bestow? The human infant arrives, indeed, more slowly at the perfect use of its senses. It arrives, also, more slowly at the perfect use of its limbs. But we never conclude because it does not rise and skip about the fields like a dropped lamb, that there is any essential difference between its muscular powers and those of other animals of creation. Why should we suppose that its vision is regulated by different laws merely because it obtains the perfect use of its eyesight somewhat later?
Let us now turn from the imperfect analysis which the sensational school presents, to the speculations of the idealist. It will be seen that the hasty conclusions of the first gave a sort of basis for the strange results to which the second would conduct us.
Kant looked in vain for the idea of extension, or of space, where the philosophers had been seeking it, in the phenomena of sensation. He pronounced, therefore, that it was not derivable from experience, did not come to us from without, through any direct communication from the senses. Not finding this idea of space where the analytical psychologist had been searching for it, he drew it at once from the mind itself. He described it as a product of the subject man, a form of the sensibility with which he invests his own sensations.
We must first remark, that to this description of what perception really is, there lies the same objection that may be urged against the account of the sensationalist. A sensation clothed in space!—is this intelligible? is it by any means an account of the matter? To invest sensation with space, is it not as if we spoke of a pleasure that was square, or of a circular pain?
So far, however, as this internal origin of the idea of space is concerned, the statement of Kant, though expressed in unusual terms, is not opposed to the general belief of mankind, or to our irresistible convictions. It may merely convey this meaning, that the mind has an immediate knowledge (drawn from the laws of its own cogitation) of space, or extension. But then, according to the universal and unalterable convictions of mankind, this idea of space, though it may be derived from the innate resources of the mind, is in fact the knowledge of an external reality—of an objective truth. Kant decided otherwise. He pronounced this form of the sensibility to be merely and only a mode of thought—that space had, in fact, no other existence, was solely a subjective truth.
This one decision has been the cause of, or at least has served as the starting-point for a series of the wildest speculations that perhaps philosophy has to record. And this decision, how arbitrary!—how dogmatic!
It must be manifest, we think, to every intelligent person, that, granting we cannot demonstrate the objective truth of the existence of space, it is equally impossible to prove its subjective nature. We cannot conceive of space but as existing really around us. The metaphysician says we may be deceived. This universal and irresistible conviction—this fundamental law of human belief, may not be correspondent with absolute truth, may not be trustworthy. Granted that we may be deceived, that there is footing here for his scepticism, he cannot proceed a step further, and show that we are deceived. When, in his turn, he would assert, or dogmatise, he at all events is as open to our scepticism as we were to his. If a fundamental belief of this kind is not to be trusted, so neither can it be convicted of falsehood. We cannot launch ourselves out of our own nature; we cannot test our own faculties of cognition. This could only be done by some superior intelligence who could survey apart the object and the percipient subject.