Philip Pullman

Well, in theory all these worlds are mutually unreachable. The physics wouldn't allow things to be otherwise. In pratice, the whole structure... leaks. Things get picked up and put down on a windowsill, for example, that opens just once, just briefly, into another world; someone passing by takes a fancy to it, and off it goes, never to be seen again. Your little pottery elephant, Marisa van Zee, here a blackbird, there a bus timetable... A small boy has an imaginary friend - they play together for hours - whispers secrets, swear eternal love, play at being king and queen... But she's not imaginary, she comes through that tumbled bit of wall behind the greenhouse, and one day he finds that someone's mended it, and she's lost for ever. Or that house you saw from the train window, that little glimpse of the perfect dwelling, and you make the same journey over and over and you never see it again. Well, that's what's hapening: an infinity of worlds, and a thousand and one little leaks in the fabric.