Mindswap

"… Marvin was trying to figure out where he stood. Was he on Earth or on its double? Was there some telltale detail that didn't match the Earth where he was born? Or perhaps several such details? Marvin searched for them for the sake of his peace of mind. He wandered around Stanhope and its surroundings, examining, exploring, and checking the flora and fauna.
Everything turned out to be in its place. Life went on its accustomed course; his father herded rat flocks, his mother, as always, peacefully laid eggs.
He set off north, to Boston and New York, then south, into the vast land of Philadelphia–Los Angeles. Everything seemed to be in order.
He thought about crossing the country from west to east under sail along the great Delaware River and continuing his investigations in the big cities of California — Schenectady, Milwaukee, and Shanghai.
However, he changed his mind, realising it was pointless to spend his life trying to determine whether he had a life worth spending.
Besides, one could suppose that even if Earth had changed, his sense organs and memory had changed too, so that nothing could be determined either way.
He lay under the familiar green sky of Stanhope and pondered this supposition. It seemed improbable. Did the giant oaks not still migrate south every year? Did the immense red sun not still sail across the sky accompanied by its dark satellite? Did the triple moons not still acquire new comets at each new moon every month? ..."