Maths has also evolved to be complex now, with the proof of four-color theorem being a computer proof. So, the statement is not as true anymore. But there was a

Maths has also evolved to be complex now, with the proof of four-color theorem being a computer proof. So, the statement is not as true anymore. But there was a short era when proving that your "static" program was correct was impossible because the number of possible combinations of input approached the size of earth (in terms of how many bits we need to represent). At the same time, almost any single mathematical theorem could be verified by a person to be correct over the course of a couple of years.