To paraphrase a famous person: the basic rules of math are so obvious, that only a fool would doubt them. They are so universal (like 3 stones will always be 3 stones), that we would not be able to process, what we see, hear, etc., if they would turn out, not to be true.
And the rest can all be created from those basic rules by logic deduction. So they have to be true, if the basic rules are true.
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