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Because an Idea from actual Sensation, and another from Memory, are very distinct Perceptions.
      §5. Secondly, Because sometimes I find, that I cannot avoid the having
those Ideas produced in my Mind. For though when my Eyes are shut,
or Windows fast, I can at Pleasure re-call to my Mind the Ideas
of Light, or the Sun, which former Sensations had lodg’d in my
Memory; so I can at pleasure lay by that 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 exteriour 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 no body who
doth not perceive the difference in himself, between contemplating
the Sun, as he hath the Idea of it in his Memory, and actually look-
ing upon it: Of which two, his perception is so distinct, that few
of his Ideas are more distinguishable one from another. And there-
fore 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.
Locke Hum IV, 11, §5, pp. 632