" Let no one read my principles who is not a mathematician,” he famously declared (less famous is the fact that the principles he was referring to were his theories of how the aortic pulmonary valve worked). Ironically, he himself was a poor mathematician, often making simple mistakes. In one of his notes he counted up his growing library: “25 small books, 2 larger books, 16 still larger, 6 bound in vellum, 1 book with green chamois cover.” This reckoning (with its charmingly haphazard system of classification) adds up to fifty, but Leonardo reached a different sum: “Total: 48,” he confidently declared. "

― Ross King , Leonardo and the Last Supper