According to RadioLab, babies recognize differences in quantities by ratios. The difference between 1 and 2 (doubling) is seen as much larger than the difference between 8 and 9. Moreover, some cultures do not see the middle of 1 and 9 as 5, but as 3. Thinking in ratios, 3 is as close to 1 as it is to 9 (multiply by three).