"[A] proposition concerning abstract ideas must be eternally true if it is ever true."
---John Locke, Essay IV.xii.14
"In any of a thousand ways he could have ceased to exist since I had the testimony of my senses for his existence... while I am alone in my study writing this I am not certain enough of this to say that I know it to be so" ---John Locke, Essay IV.xi.9: I'm theorizing that this is the origin of Descartes' automata-with-umbrellas example.
"I can be as sure that if I move thus and so I will feel pain as I can be that I shall move thus and so..."
---John Locke, Essay IV.xi.8
---John Locke, Essay IV.xi.6
[The above come from the Jonathan Bennett translation].