"It is the mark of an educated mind to be able to entertain a thought without accepting it." Aristotle

Not confidence, acceptance.

The second isn’t an assertion, it’s a hypothetical.

Eg, “let’s imagine that elves are real” is distinct from “elves are real”.

It's a confidence-modulated assertion. Hypothesis is low confidence. Fact is high confidence.

There is no magic point at which hypothesis suddenly becomes fact. It's simply a matter of confidence.

It’s confidence independent:

“Let X” is a context bound hypothetical — we’re assuming it is true within some argument; we have perfect confidence in the hypothetical, but only within that context.

That’s distinct from an assertion, which is a statement about how the world is with some associated confidence.

Have you never seen proofs by contradiction?