There is some indication that the number system at Uruk did not simply "evolve", but was invented by a now forgotten mathematical genius. The Uruk tablets not only use a base 10 number system (the "finger" system), but they also employ a base 60 system. Very large numbers are expressed in multiples of 60, represented by a large wedge. Ten times sixty, 600 was a large wedge punched with the symbol for ten. Why not simply use powers of ten? Otto Neugebauer, the great historian of ancient mathematics, argues that the base 60 was used to make fractions easy to calculate: 60 is divisible in thirds as well and halves. Base 60 made mental division easier. While six is also divisible in thirds as well as halves, it may have been too close to 10. The influence of the ancient Uruk mathematician has remained with us to this day in our measures of time: the hour is divided into 60 minutes, and the minute is divided into sixty seconds.