3

This number is a prime.

The law of proportions, called "Rule of Three" by the Indian mathematician Brahmagupta (598-668), became a standard of rational thought. For example, Abraham Lincoln wrote that as a young man he "could read, write, and cipher to the Rule of Three." Charles Darwin wrote "I have no faith in anything short of actual measurement and the Rule of Three." Perhaps less known is the fact that they were born on the exact same day (February 12, 1809).