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But when we pass beyond the condition of the organ as itself unknown to the mind and outside of it, and contemplate the true immaterial content of consciousness, the case is, if possible, still plainer. Perception as an act of mind does not reveal to us the instrument of sense employed, or the state of that instrument. The connection between a mental state and the physical state which accompanies it, is mysterious and unknown; it is not so much as hinted at in the very act of perception in consciousness. For aught that we can see, the last might be very different from what it is, and the first remain the same. Indeed, that there are to sight and hearing accompanying physical states, what these states are, and even where they are, constitute facts which require to be learned from experience.

It has helped to obscure the doctrine of perception, that a distinct image on the retina has been found to intervene between the object and vision. This image has drawn attention as something especially necessary and explanatory. It is, however, in reference to vision a mere accident, as much so as the possible images which may be formed within the tube of a telescope. These images one and all play no part as images, but simply as causal links. The image on the retina is only one term in the physical agencies, which finally express themselves in the molecular changes of the brain. If we were to examine the eye in delirium, we should find no images in it of the objects evoked by the mind.

Even in advanced life we do not always recognize at which ear a given sound chiefly enters, and tentatively test the question by turning the attention first in one direction, then in the other. The content of consciousness, then, is not of such a nature as to reveal in perception the states of the retina, or of the auditory nerve; or whether there is in them more or less of foreign action. These changes are