There are at least two people in London with exactly the same number of hairs on their heads, not counting the bald, who would make it too easy. No head has anywhere near a million hairs and London has millions of people with hair, so there are more heads than possible counts, and some count has to be used twice. This has been known since at least the 1620s, when it turned up in a Jesuit’s book of mathematical propositions. It is certain, and it will never help you find them: nobody has counted, nobody will, and the proof doesn’t point at anyone. Most of what we know, we know from having seen an example. This is knowing with no example at all, a fact about two strangers who will go their whole lives unaware they are the answer.