Most people know how to count, but the way we master this ability remains something of a puzzle. Numerals were invented only around four to five thousand years ago, meaning it is unlikely that enough time has elapsed for specialized parts of the brain for processing numbers to evolve, which suggests that math is largely a cultural invention. It appears to be based on an interface between vision and reasoning that we share with other animals, allowing us to "see" small numbers—up to around five—without counting. This ability—often called ’the number sense’—lays the foundations of later mathematical knowledge, but its basis is poorly understooD、It has been argued that the number sense itself may be innate, but this fails to account for why learning to master the use of small numbers is such a difficult and drawn-out process in children.