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, and have an existence distinct and separate
from them: so that the question is not now concerning the same numerical
ideas, but whether there be any one and the same sort of species of ideas
equally perceivable to both senses; or, in other words, whether
extension, figure, and motion perceived by sight are not specifically
distinct from extension, figure, and motion perceived by touch.

122. But before I come more particularly to discuss this matter, I find
it proper to consider extension in abstract: for of this there is much
talk, and I am apt to think that when men speak of extension as being an
idea common to two senses, it is with a secret supposition that we can
single out extension from all other tangible and visible qualities, and
form thereof an abstract idea, which idea they will have common both to
sight and touch. We are therefore to understand by extension in abstract
an idea of extension, for instance, a line or surface entirely stripped
of all other sensible qualities and circumstances that might determine it
to any particular existence; it is neither black nor white, nor red, nor
hath it any colour at all, or any tangible quality whatsoever and
consequently it is of no finite determinate magnitude: for that which
bounds or distinguishes one extension from another is some quality or
circumstance wherein they disagree.