Two Black Maidens

Two black maidens set their minds to proof...
They do their math, and prove.
Calculus, they use, to prove Pythagorean Theorem,
Yet Calculus is proven so much, so
That Pythagorean Theorem is proven too!

It is a biconditional,
Two Tautologies that necessitate.
The very crux of Equalities,
And the very crux of all mathematics.
It is how, oh my souls,
That science knows.