The technical term often used to describe animals’ judgement of numbers is the approximate number system. What it does not provide is precision. It shows — and this is the same in every species tested — a characteristic pattern of errors, with discrimination becoming less accurate as the quantities get bigger. Rhesus monkeys can tell one from two, two from three, three from four, four from five ... but start to fail from five upwards. Rats that learned to press a lever a given number of times, from four up to twenty-four, became markedly less and less precise in their responses as the number increased: by the top end of the range they would merely produce a spread of numbers around the target. It is a common observation that when testing the accuracy of animals’ number sense, the size of the numbers matters.