I think you can argue that there are fundamentally abstract concepts we know to be true, such as 1+1=2. I suppose this can be classified as circular reasoning by realizing '2' is the object we implicitly define to possess a value of twice that of '1,' so to say 1 + 1 = 2 is using the definition we've 'arbitrarily' come up with to circularly create a "truth." [...]
Hi swagmaster,
yes. ( as far as I understood. )
And / but that's why we find maths in logic, too! Look:
https://en.wikipedia.org/wiki/Universal_quantificationSome problems there seem to exist, though.
https://en.wikipedia.org/wiki/Allegory_of_the_CaveHow can we be sure, that it, what we assume to BE, really IS ? Perhaps it's only something else, in disguise? (Ok, a mathematical number will have difficulties to disguise / mask itself, if it doesn't want to have its true nature be detected / revealed by us ....

)
But imagine, you go camping with friends, in the dark woods, and suddenly this figure leaps from behind a tree, right into the middle of your campfire-evening!

https://de.wikipedia.org/wiki/Jersey_DevilAnd it snarls angrily at you all!!

But then, after a while, staring at the anxious spectators, it says: "Give me a beer, too, I'm thirsty!", and then it takes off its mask, and reveals to be the father of one of your friends, only having played a trick on you! Everybody laughs!

___
Or, imagine this little modification of the scene:
The figure this time leaps from behind the trees, and snarls!
Everybody thinks: "hahaha, it's just a joke!" But then it grabs one of you (one who laughed very loud) , and flaps its wings, and flies away with him / her.
Nobody ever saw both of them again.
And now, nobody laughs any longer.
___
We would conclude, in my opinion: What we assume to BE, might not always be REAL...
But perhaps it's valid for this assumption, too?

Very cordially, 8_octaves!