48.  One reason of which seems to be this.  It is thought a great absurdity to imagine that one and the same thing should have any more than one extension, and one figure.  But the extension and figure of a body, being let into the mind two ways, and that indifferently either by sight or touch, it seems to follow that we see the same extension and the same figure which we feel.

49.  But if we take a close and accurate view of things, it must be acknowledged that we never see and feel one and the same object.  That which is seen is one thing, and that which is felt is another.  If the visible figure and extension be not the same with the tangible figure and extension, we are not to infer that one and the same thing has divers extensions.  The true consequence is that the objects of sight and touch are two distinct things.  It may perhaps require some thought rightly to conceive this distinction.  And the difficulty seems not a little increased, because the combination of visible ideas hath constantly the same name as the combination of tangible ideas wherewith it is connected:  which doth of necessity arise from the use and end of language.

50.  In order therefore to treat accurately and unconfusedly of vision, we must bear in mind that there are two sorts of objects apprehended by the eye, the one primarily and immediately, the other secondarily and by intervention of the former.  Those of the first sort neither are, nor appear to be, without the mind, or at any distance off; they may indeed grow greater or smaller, more confused, or more clear, or more faint, but they do not, cannot approach or recede from us.  Whenever we say an object is at a distance, whenever we say it draws near, or goes farther off, we must always mean it of the latter sort, which properly belong to the touch, and are not so truly perceived as suggested by the eye in like manner as thoughts by the ear.

51.  No sooner do we hear the words of a familiar language pronounced in our ears, but the ideas corresponding thereto present themselves to our minds:  in the very same instant the sound and the meaning enter the understanding:  so closely are they united that it is not in our power to keep out the one, except we exclude the other also.  We even act in all respects as if we heard the very thoughts themselves.  So likewise the secondary objects, or those which are only suggested by sight, do often more strongly affect us, and are more regarded than the proper objects of that sense; along with which they enter into the mind, and with which they have a far more strict connexion, than ideas have with words.  Hence it is we find it so difficult to discriminate between the immediate and mediate objects of sight, and are so prone to attribute to the former what belongs only to the latter.  They are, as it were, most closely twisted, blended, and incorporated together.  And the prejudice is confirmed and riveted in our thoughts by a long tract of time, by the use of language, and want of reflexion.  However, I believe anyone that shall attentively consider what we have already said, and shall say, upon this subject before we have done (especially if he pursue it in his own thoughts) may be able to deliver himself from that prejudice.  Sure I am it is worth some attention, to whoever would understand the true nature of vision.