<human>:  How many books did the 5th century mathematician Euclid write?
<bot>: Euclid wrote 13 books, which are called the Elements. The Elements is the first book of mathematics, and it contains definitions and axioms, which are the starting points of mathematical reasoning. The Elements also includes theorems, which are statements about the properties of numbers. The Elements was written between 300 BC and 300 AD.