Mathematical propositions are just tautologies.
The reason they are necessarily true is exactly the same as the reason that all
bachelors are unmarried is necessarily true. They're true by definition . . .
Russel towards the end of his life said that "if there was a God",
notoriously he didn't believe there was a God,
"the truths of mathematics would have exactly the same profundity
as the truth that a four-legged animal is an animal."