Take a dozen of 1911 Napiers turned out of the same shops, on the same pattern, with the same parts, and of the same horse power, and driven with the same make of petrol. Not all will last the same time, not all will wear as well; one will run 20 miles to the gallon, another only 16. One is in the shops every few months, the other is as good as new at the end of the year. And it is not a matter of the chauffeur alone, in the way the machine is driven, or the care which is taken, there are more curious differences in spite of the similarity, indeed of the identity of the machines. And the same is true of men.
Dr. William Osler (1849-1919) as quoted by Michael Bliss in William Osler: A Life of Medicine.