There was once upon a time a census officer who had to record the names of all householders in a certain Welsh village. The first that he questioned was called William Williams; so were the second, third, fourth.... At last he said to himself: 'This is tedious; evidently they are all called William Williams. I shall put them down so and take a holiday'. But he was wrong; there was just one whose name was John Jones. This shows that we may go astray if we trust too implicitly to induction by simple enumeration.