A perfect number is a natural number which is equal to the sum of its integer divisors including 1 and excluding itself, but a number n is also perfect in which the sum of its divisors including 1 and itself is equal to 2n. The natural numbers are infinite, for each of them there is a successor number and if it will never be possible to know how many, among the natural numbers, there can be perfect numbers, it is possible to know why there are even perfect numbers and there cannot be odd perfect numbers .The perfect number equal to 2n recalls a measurement technique, used 35,000 years ago when numbers were not known and which is similar to today's one-to-one correspondence. The correspondence of years ago consisted in associating each element of a set A with an element of set B; a concrete correspondence today is: "in a shirt the A.soles can be associated with the B.brass". Years ago, not knowing how to count, any set A was made to correspond to a set B in order to obtain that any difference between the two sets was the confirmation or not that the two sets were equal.