Once a book falls into our possession, it is ours, the same way children lay their claim: 'That's my book.' As if it were organically part of them. That must be why we have so much trouble returning borrowed books. It's not exactly theft (of course not, we're not thieves, what are you implying?); it's simply a slippage in ownership or, better still, a transfer of substance. That which belonged to someone else becomes mine when I look at it. And if I like what I read, naturally I'll have difficulty giving it back.