For, since A is composite, some number will measure it.
Let a number measure it, and let it be B.
Now, if B is prime, what was enjoined will have been done.
But if it is composite, some number will measure it.
Let a number measure it, and let it be C.
Then, since C measures B, and B measures A,
therefore C also measures A.
And, if C is prime, what was enjoined will have been done.
But if it is composite, some number will measure it.
Thus, if the investigation be continued in this way, some prime number will be found which will measure the number before it, which will also measure A.
For, if it is not found, an infinite series of numbers will measure the number A,
each of which is less than the other: which is impossible in numbers.
Therefore some prime number will be found which will measure the one before it, which will also measure A.
Complete text
Book I