if we suppose a third eye C from a due distance to behold the fund
of A, then indeed the things projected thereon shall, to C, seem pictures
or images in the same sense that those projected on B do to A.

119. Rightly to conceive this point we must carefully distinguish between
the ideas of sight and touch, between the visible and tangible eye; for
certainly on the tangible eye nothing either is or seems to be painted.
Again, the visible eye, as well as all other visible objects, hath been
shown to exist only in the mind, which perceiving its own ideas, and
comparing them together, calls some PICTURES in respect of others. What
hath been said, being rightly comprehended and laid together, doth, I
think, afford a full and genuine explication of the erect appearance of
objects; which phenomenon, I must confess, I do not see how it can be
explained by any theories of vision hitherto made public.

120. In treating of these things the use of language is apt to occasion
some obscurity and confusion, and create in us wrong ideas; for language
being accommodated to the common notions and prejudices of men, it is
scarce possible to deliver the naked and precise truth without great
circumlocution, impropriety, and (to an unwary reader) seeming
contradictions; I do therefore once for all desire whoever shall think it
worth his while to understand what I have written concerning vision, that
he would not stick in this or that phrase, or manner of expression, but
candidly collect my meaning from the whole sum and tenor of my discourse,
and laying aside the words as much as possible, consider the bare notions
themselves, and then judge whether they are agreeable to truth and his
own experience, or no.

121. We have shown the way wherein the mind by mediation of visible ideas
doth perceive or apprehend the distance, magnitude and situation of
tangible objects. We come now to inquire more particularly concerning the
difference between the ideas of sight and touch, which are called by the
same names, and see whether there be any idea common to both senses. From
what we have at large set forth and demonstrated in the foregoing parts
of this treatise, it is plain there is no one selfsame numerical
extension perceived both by sight and touch; but that the particular
figures and extensions perceived by sight, however they may be called by
the same names and reputed the same things with those perceived by touch,
are nevertheless different,