T idea, and take into my view that of the smell of a rose, or taste
of sugar. But, if I turn my eyes at noon towards the sun, I cannot avoid
the ideas which the light or sun then produces in me. So that there is a
manifest difference between the ideas laid up in my memory, (over which,
if they were there only, I should have constantly the same power to
dispose of them, and lay them by at pleasure,) and those which force
themselves upon me, and I cannot avoid having. And therefore it must
needs be some exterior cause, and the brisk acting of some objects
without me, whose efficacy I cannot resist, that produces those ideas
in my mind, whether I will or no. Besides, there is nobody who doth not
perceive the difference in himself between contemplating the sun, as
he hath the idea of it in his memory, and actually looking upon it: of
which two, his perception is so distinct, that few of his ideas are
more distinguishable one from another. And therefore he hath certain
knowledge that they are not BOTH memory, or the actions of his mind, and
fancies only within him; but that actual seeing hath a cause without.

6. III. Thirdly, Because Pleasure or Pain, which accompanies actual
Sensation, accompanies not the returning of those Ideas without the
external Objects.

Add to this, that many of those ideas are PRODUCED IN US WITH PAIN,
which afterwards we remember without the least offence. Thus, the pain
of heat or cold, when the idea of it is revived in our minds, gives us
no disturbance; which, when felt, was very troublesome; and is again,
when actually repeated: which is occasioned by the disorder the external
object causes in our bodies when applied to them: and we remember the
pains of hunger, thirst, or the headache, without any pain at all; which
would either never disturb us, or else constantly do it, as often as we
thought of it, were there nothing more but ideas floating in our minds,
and appearances entertaining our fancies, without the real existence
of things affecting us from abroad. The same may be said of PLEASURE,
accompanying several actual sensations. And though mathematical
demonstration depends not upon sense, yet the examining them by diagrams
gives great credit to the evidence of our sight, and seems to give it a
certainty approaching to that of demonstration itself. For, it would be
very strange, that a man should allow it for an undeniable truth, that
two angles of a figure, which he measures by lines a