that planes are no more the immediate
object of sight than solids. What we strictly see are not solids, nor yet
planes variously coloured: they are only diversity of colours. And some
of these suggest to the mind solids, and other plane figures, just as
they have been experienced to be connected with the one or the other: so
that we see planes in the same way that we see solids, both being equally
suggested by the immediate objects of sight, which accordingly are
themselves denominated planes and solids. But though they are called by
the same names with the things marked by them, they are nevertheless of a
nature entirely different, as hath been demonstrated.

159. What hath been said is, if I mistake not, sufficient to decide the
question we proposed to examine, concerning the ability of a pure spirit,
such as we have described, to know GEOMETRY. It is, indeed, no easy
matter for us to enter precisely into the thoughts of such an
intelligence, because we cannot without great pains cleverly separate and
disentangle in our thoughts the proper objects of sight from those of
touch which are connected with them. This, indeed, in a complete degree
seems scarce possible to be performed: which will not seem strange to us
if we consider how hard it is for anyone to hear the words of his native
language pronounced in his ears without understanding them. Though he
endeavour to disunite the meaning from the sound, it will nevertheless
intrude into his thoughts, and he shall find it extreme difficult, if not
impossible, to put himself exactly in the posture of a foreigner that
never learned the language, so as to be affected barely with the sounds
themselves, and not perceive the signification annexed to them.

160. By this time, I suppose, it is clear that neither abstract nor
visible extension makes the object of geometry; the not discerning of
which may perhaps have created some difficulty and useless labour in
mathematics. Sure I am, that somewhat relating thereto has occurred to my
thoughts, which, though after the most anxious and repeated examination I
am forced to think it true, doth, nevertheless, seem so far out of the
common road of geometry, that I know not whether it may not be thought
presumption, if I should make it public in an age, wherein that science
hath received such mighty improvements by new methods; great part whereof,
as well as of the ancient discoveries, may perhaps lose their reputation,
and much of that