which they followed before such
change; and, further, when afterwards they impinge on the new
surfaces by their own spontaneous movement, they will be
refracted in the same manner, as though they had been impelled
towards those surfaces by external bodies; consequently, they
will, while they continue to be thus refracted, affect the human
body in the same manner, whereof the mind (II. xii.) will again
take cognizance--that is (II. xvii.), the mind will again regard
the external body as present, and will do so, as often as the
fluid parts of the human body impinge on the aforesaid surfaces
by their own spontaneous motion. Wherefore, although the
external bodies, by which the human body has once been affected,
be no longer in existence, the mind will nevertheless regard them
as present, as often as this action of the body is repeated.
Q.E.D.

Note.--We thus see how it comes about, as is often the case,
that we regard as present many things which are not. It is
possible that the same result may be brought about by other
causes; but I think it suffices for me here to have indicated
one possible explanation, just as well as if I had pointed out
the true cause. Indeed, I do not think I am very far from the
truth, for all my assumptions are based on postulates, which
rest, almost without exception, on experience, that cannot be
controverted by those who have shown, as we have, that the human
body, as we feel it, exists (Coroll. after II. xiii.).
Furthermore (II. vii. Coroll., II. xvi. Coroll. ii.), we clearly
understand what is the difference between the idea, say, of
Peter, which constitutes the essence of Peter's mind, and the
idea of the said Peter, which is in another man, say, Paul. The
former directly answers to the essence of Peter's own body, and
only implies existence so long as Peter exists; the latter
indicates rather the disposition of Paul's body than the nature
of Peter, and, therefore, while this disposition of Paul's body
lasts, Paul's mind will regard Peter as present to itself, even
though he no longer exists. Further, to retain the usual
phraseology, the modifications of the human body, of which the
ideas represent external bodies as present to us, we will call
the images of things, though they do not recall the figure of
things. When the mind regards bodies in this fashion, we say
that it imagines. I will here draw attention to the fact, in
order to indicate where error lies, that the imagina