Page "Pseudorandom number generator" Paragraph 14

Von Neumann judged hardware random number generators unsuitable, for, if they did not record the output generated, they could not later be tested for errors.

If they did record their output, they would exhaust the limited computer memories available then, and so the computer's ability to read and write numbers.

If the numbers were written to cards, they would take very much longer to write and read.

On the ENIAC computer he was using, the " middle square " method generated numbers at a rate some hundred times faster than reading numbers in from punched cards.