We bring the matter to the test of actual experiment.  We make certain experiments, seriatim, upon each of the items that lie within the sentient sphere, and we note the effect which each experiment has upon that portion of the contents which is not meddled with.  In the exercise of vision, for example, we remove a book, and no change is produced in our perception of a house; a cloud disappears, yet our apprehension of the sea and the mountains, and all other visible things, is the same as ever.  We continue our experiments, until our test happens to be applied to one particular phenomenon, which lies, if not directly, yet virtually, within the sphere of vision.  We remove or veil this small visual phenomenon, and a totally different effect is produced from those that took place when any of the other visual phenomena were removed or veiled.  The whole landscape is obliterated.  We restore this phenomenon—­the whole landscape reappears:  we adjust this phenomenon differently—­the whole landscape becomes differently adjusted.  From these experiments we find, that this phenomenon is by no means an ordinary sensation, but that it differs from all other sensations in this, that it is the sense itself appearing in the form of a sensation.  These experiments prove that it is the sense itself, and nothing else, which reveals itself to us in the particular phenomenon the eye.  If experience informed us that the particular adjustment of some other visual phenomenon (a book, for instance) were essential to our apprehension of all the other phenomena, we should, in the same way, be compelled to regard this book as our sense of sight manifested in one of its own sensations.  The book would be to us what the eye now is:  it would be our bodily organ:  and no a priori reason can be shown why this might not have been the case.  All that we can say is, that such is not the finding of experience.  Experience points out the eye, and the eye alone, as the visual sensation essential to our apprehension of all our other sensations of vision, and we come at last to regard this sensation as the sense itself.  Inveterate association leads us to regard the eye, not merely as the organ, but actually as the sense of vision.  We find from experience how much depends upon its possession, and we lay claim to it as a part of ourselves, with an emphasis that will not be gainsaid.

An interesting enough subject of speculation would be, an enquiry into the gradual steps by which each man is led to appropriate his own body.  No man’s body is given him absolutely, indefeasibly, and at once, ex dono Dei.  It is no unearned hereditary patrimony.  It is held by no a priori title on the part of the possessor.  The credentials by which its tenure is secured to him, are purely of an a posteriori character; and a certain course of experience must be gone through before the body can become his.  The man acquires it, as he does originally all other property, in a certain