Eratosthenes (c. 285–c. 205B.C.E.) was a Greek mathematician and philosopher who devised a method to identify (or “sift” out) prime numbers from a list of natural numbers arranged in order. It is a simple method, although it becomes tedious to identify large prime numbers. The steps of the sieve are:

- Write all natural numbers in order, omitting 1.
- Circle the number 2 and then cross out every other number. Every second number will be a multiple of 2 and hence is not a prime number.
- Circle the number 3 and then cross out every third number which will be a multiple of 3 and, therefore, not a prime number.
- The numbers that are circled are prime and those that are crossed out are composite numbers.