Visible extension, then, or the perception of colours external to colours, being admitted, Mr Bailey's argument, if he still adheres to it, must be presented to us in this form. He must maintain that the theory requires that the objects of touch should not only be suggested by the visual objects with which they have been associated, but that they should actually be _seen_. And then he must maintain that no power of association can enable us to see an object which can only be touched--a position which, certainly, no one will controvert. The simple answer to all which, is, that we never do see tangible objects--that the theory never requires we should, and that no power of association is necessary to account for a phenomenon which never takes place.
We cannot help thinking, that not a little of the misconception on this subject which prevails in the writings of Mr Bailey, and, we may add, of many other philosophers, originates in the supposition that we identify vision with the eye in the mere act of seeing, and in their taking it for granted that sight of itself informs us that we possess such an organ as the eye. Of course, if we suppose that we know instinctively, or intuitively, from the mere act of seeing, that the eye is the organ of vision, that it forms a part of the body we behold, and is located in the head, it requires no conjurer to prove that we _must_ have an instinctive, or intuitive, knowledge of visible things as larger than that organ, and, consequently, as external to it. In this case, no process of association is necessary to account for our knowledge of the distance of objects. That knowledge must be directly given in the very function and exercise of vision, as every one will admit, without going to the expense of an octavo volume to have it proved.
But we hold that no truth in mental philosophy is more incontestable than this, that the sight originally, and of itself, furnishes us with no knowledge of the eye, as we _now_ know that organ to exist. It does not inform us that we have an eye at all. And here we may hazard an observation, which, simple as it is, appears to us to be new, and not unimportant in aiding us to unravel the mysteries of sensation; which observation is, that, in no case whatever, does any sense inform us of the existence of its appropriate organ, or of the relation which subsists between that organ and its objects, but that the interposition of some other sense[36] is invariably required to give us this information. This truth, which we believe holds good with regard to all the senses, is most strikingly exemplified in the case of vision, as we shall now endeavour to illustrate.