[OUR KNOWLEDGE OF THE EXISTENCE OF OTHER THINGS - JOHN LOCKE - FROM AN ESSAY CONCERNING HUMAN UNDERSTANDING]
IT is the actual receiving of ideas from without that gives us notice of the existence of other things, and makes us know that something does exist at that time without us which causes that idea in us, though perhaps we neither know nor consider how it does it. And this, though not so certain as our own intuitive knowledge, or as the deductions of our reason employed about the clear abstract ideas of our own minds, yet deserves the name of knowledge.
It is plain that those perceptions are produced by exterior causes affecting our senses for the following reasons:
Because those that lack the physical organs of any sense never can have the ideas belonging to that sense produced in their minds.
Because sometimes I find I cannot avoid having those ideas produced in my mind; for as when my eyes are shut, or the windows fast, I can at pleasure recall to my mind the ideas of light or the sun which former sensations have lodged in my memory; so I can at pleasure lay by that idea and take into my view that 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 produces in me. There is nobody who does not perceive the difference in himself contemplating the sun as he has an 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 has certain knowledge that they are not both memory or the action of his mind and fancies only within him, but that actual seeing has a cause without.
Add to this that many of those ideas are produced with pain, which afterwards we remember without the least offence.
Lastly, our senses bear witness to the truth of each other's report concerning the existence of sensible things without us and around us.
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