We are assured by optical metaphysicians, or metaphysical opticians, that, in the operations of vision, we never get beyond the eye itself, or the representations that are depicted therein.  We see nothing, they tell us, but what is delineated within the eye.  Now, the way in which a plain man should meet this statement, is this—­he should ask the metaphysician what eye he refers to.  Do you allude, sir, to an eye which belongs to my visible body, and forms a small part of the same; or do you allude to an eye which does not belong to my visible body, and which constitutes no portion thereof?  If the metaphysician should say, that he refers to an eye of the latter description, then the plain man’s answer should be—­that he has no experience of any such eye—­that he cannot conceive it—­that he knows nothing at all about it—­and that the only eye which he ever thinks or speaks of, is the eye appertaining to, and situated within, the phenomenon which he calls his visible body.  Is this, then, the eye which the metaphysician refers to, and which he tells us we never get beyond?  If it be—­why, then, the very admission that this eye is a part of the visible body, (and what else can we conceive the eye to be?) proves that we must get beyond it.  Even supposing that the whole operation were transacted within the eye, and that the visible body were nowhere but within the eye, still the eye which we invariably and inevitably fill in as belonging to the visible body, (and no other eye is ever thought of or spoken of by us,)—­this eye, we say, must necessarily exclude the visible body, and all other visible things, from its sphere.  Or, can the eye (always conceived of as a visible thing among other visible things) again contain the very phenomenon (i.e. the visible body) within which it is itself contained?  Surely no one will maintain a position of such unparalleled absurdity as that.

The science of optics, in so far as it maintains, according to certain physiological principles, that in the operation of seeing we never get beyond the representations within the eye, is founded on the assumption, that the visible body has no visible eye belonging to it.  Whereas we maintain, that the only eye that we have—­the only eye we can form any conception of, is the visible eye that belongs to the visible body, as a part does to a whole; whether this eye be originally revealed to us by the touch, by the sight, by the reason, or by the imagination.  We maintain, that to affirm we never get beyond this eye in the exercise of vision, is equivalent to asserting, that a part is larger than the whole, of which it is only a part—­is equivalent to asserting, that Y, which is contained between X and Z, is nevertheless of larger compass than X and Z, and comprehends them both.  The fallacy we conceive to be this, that the visible body can be contained within the eye, without the eye of the visible body also being contained therein.  But this is a procedure, which no law,