so then euclid goes one step further. and that's the point. having identified the primes, we can now ask questions like, how many of them are there? and what euclid proved is that there are an infinite number. so let's see how he did it. euclid's proof of the infinitude of primes shows that if we multiply together any finite collection of primes and then add 1, the resulting sum is not divisible by any of the primes that we start with. for example, multiply the prime numbers 2, 3, and 5 and then add 1. the result is 31, a number which can't be divided by either 2, 3, or 5. but in fact, 31 is itself prime, so we've discovered a new prime number! now, if we continue down this path and use the primes 2, 3, 5, and 31 to create a new number, this time we get 931, which also cannot be factored by any of the primes we used to create it. as it turns out, 931 is not a prime. it can be divided by both 7 and 19. but guess what, we've just discovered two new primes. so even if the output of euclid's formula is not itself a prime number, any of its factors