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 and angles of a diagram, should be bigger one than the other, and yet doubt of the existence of those lines and angles, which by looking on he makes use of to measure that by.

7.  IV.  Fourthly, Because our Senses assist one another’s Testimony of the Existence of outward Things, and enable us to predict.

Our senses in many cases bear witness to the truth of each other’s report, concerning the existence of sensible things without us.  He that sees a fire, may, if he doubt whether it be anything more than a bare fancy, feel it too; and be convinced, by putting his hand in it.  Which certainly could never be put into such exquisite pain by a bare idea or phantom, unless that the pain be a fancy too:  which yet he cannot, when the burn is well, by raising the idea of it, bring upon himself again.