`One day in a grammar school maths lesson I got into a crying jag over the
notion of minus numbers. Minus one threw out my universe, it couldn't exist,
I couldn't understand it. This, I realised tearfully, under coaxing from an
amused (and mildly amazed) teacher, was because I thought numbers were
*things*. In fact, cabbages. We'd been taught in Miss Myra's class to do
addition and subtraction by imagining more cabbages and fewer cabbages.
Every time I did mental arithmetic I was juggling ghostly vegetables in my
head. And when I tried to think of minus one I was trying to imagine an
anti-cabbage, an anti-matter cabbage, which was as hard as conceiving of an
alternative universe.