Human Knowledge

Not only human but also animal knowledge is from sense inputs, teaching and self indulgence.

Teaching - ability to gain knowledge from others sense inputs and self indulgence. Trusting, believing concepts taught by others makes such concepts universal.

Mathematical concepts need sense input initially to be formed. like looking at marbles to understand counting. After the brain forms the concept it dishes the knowledge of sense inputs - kicking out the ladder after you climbed. Similarly brain may be forgetting what was taught from external sources after forming the concept in the brain.
So a mature brain will feel these concepts inside it's brain. But a young child may have the connection to sense inputs. Children learn by pictures adults learn by reading. pictures are more near to sense inputs.

Trusted teacher - when you trust someone say your parents and when you are young, what is taught registered in the brain more easily and permanently. After a while you will not remember who taught but the thought will be in the brain as if it was of your own. Kicking your the ladder after you climbed. Tractatus-Wittgenstein.