The author is making the following reasoning: Conjecture 1 and Conjecture 2 can not both be true. Conjecture 1 is believed to be true. Therefore Conjecture 2 is

The author is making the following reasoning: Conjecture 1 and Conjecture 2 can not both be true. Conjecture 1 is believed to be true. Therefore Conjecture 2 is false, which implies that there must exist a heavy cluster of primes out there.