I do not understand the animosity you have towards for merely posting a quote which I found interesting from a book on paradoxes I was reading.
The London Casebook Of Detective Rene Descartes - Chapter 1
The acrid scent of stale cigarette smoke hung wearily in the air of the dingy Whitehall office. The only sound was the querulous buzz of a prying bluebottle indolently hopping among the dun box files clustered above the fireplace occupied by the regulation Scotland Yard electric fire, one bar of which flickered hesitantly in a perfunctory attempt to warm the November gloom.
Detective-Inspector Rene `Doubty' Descartes absent-mindedly flicked grey-white ash from the sleeve of his only vicuna jacket and stared moodily across the pigeon-violated rooftops of Whitehall. `I muse,' he thought. `Therefore. . . .'
The ginger telephone shrilled its urgent demand. Descartes, rudely awakened from his reverie, snatched the receiver to his ear.
`Descartes here,' he posited.
`Sorry to interrupt, sir.' The familiar tones of Sergeant Warnock floated down the line. `Sergeant Warnock here.'
`How can you be sure?'
`I think I am Sergeant Warnock, therefore I am Sergeant Warnock,' replied Sergeant Warnock confidently. Some of Doubty's thinking was beginning to rub off.
`But if you thought you were Marcus Aurelius would you therefore be Marcus Aurelius?' parried the forensic savant deftly.
`Er . . . probably not,' admitted the trusty sergeant, chancing his arm. When the Detective-Inspector was in moods like this, routine business could take days.
`So simply because you think you are Sergeant Warnock, it does not necessarily follow that you are,' his postulate continued.
`But, sir, you said, "You think something therefore you are something".'
`No, no, sergeant, you haven't got it at all.'
`Well, sir,' the stalwart sergeant gamely countered, `there must be a strong probability that I am Sergeant Warnock. Couldn't we on this occasion proceed on that assumption.'
`I'm afraid that it is this "beyond all reasonable doubt" philosophy that has bedeviled the reputation of police thinking since the days of that woolly pragmatist Peel.'
`But this is an urgent matter, sir. The Prime Minister is on the other line.'
`My dear putative sergeant, this problem of your identity is something we are going to have to sort out sooner or later.'
`But it's the Prime Minister, sir.'
`But how do we know it's the Prime Minister?'
`This is a perfect illustration of my theme, Warnock . . .'
`. . . if that is indeed to whom I am speaking. If I cannot be sure of the Warnockness of the person or apparent person with whom I am at present speaking, how _a fortiori_ can I accept an authentication from this source of a third party of whom my direct and verifiable experience is even further removed?'
`He's rung off anyway, sir.'
`If indeed he was ever there.'
`Well if he was, sir, then he almost definitely asked you to call him back. Can I get him for you, sir?'
`Not so fast, sergeant, for I will assume for the moment that that is who you are.'
`Thank you very much, sir.'
`If I now call the Prime Minister, how is he for certain to know that he is speaking to me?'
`Ah but that's his problem, sir.'
`But how shall I know that I am speaking to him?'
`You're calling him, sir.'
`But suppose I speak to someone, thinking him to be the Prime Minister when in fact he is not; then the Prime Minister will be disclosing what may well be state secrets to another party, believing him to be me.'
`But surely, sir, just because you're speaking to a third party it does not follow as a necessary consequence that the P.M. is speaking to anyone at all.'
Descartes sucked thoughtfully at his familiar thumb. `. . . Good work, sergeant. Get him _toute suite_.' Then replacing the receiver he ruefully swung round on the familiar leather trapeze and stared wistfully out of the window. `Funny old London,' he thought. At least the pouring rain had stopped. Or rather, it certainly seemed there was no entity _a_, such that `_x_ is rainy and pouring' was true when _x_ was _a_, but not otherwise.