Readers who remember their A.A. Milne from childhood, not to mention those who periodically revisit the Pooh books in search of good humour and attractive writing, will recall the little ritual enacted by Christopher Robin in the poem ‘Lines and Squares’: this involved treading only on the paving-slabs, never on the cracks between them, for the excellent reason that those who stepped on the ‘lines’ rather than the ‘squares’ would be consumed by ravenous bears. The one drawback with this, I decided at the time, was that it might just as well be the other way round; and so, as a quite small child, for whom the stretch might be considerable, I resolved to tread firmly on each line, which was all very well until the paving-slabs gave way to a stretch of solid concrete or tarmac or (more often than not, since we lived in the country) grass verge. In either case, however, the principle remains the same: a child presented with a pair of alternatives, such as lines and squares, will instinctively nominate one as benign and safe, the other as evil and dangerous. I seem to have applied this obsessively to almost everything. And as soon as I was old enough to have heard the word and understood roughly what it meant, I grandly named this habit of mind ‘The Oppositions’.

For instance, when I was four or five years old, I used to catch the green double-decker London Transport RT bus home from ...