The reason why language and math works is because [b]A=A[/b] and [b]1=1[/b]. All parties have to assume that this is true or it doesn't work. Now if your friend was talking about a completely different language he could be right ("tree" in russian is "дерево") but this is exactly why dictionaries exist.
The phrase "The proof is the burden of the accuser." (or something) comes to mind. He's making baseless claims.
English
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Again, every time I tell him if he is trying to prove something, or like when he said, you can't prove that I don't see infrared, I state this, and he says, I don't have to prove anything, blah blah, so then the argument ends because he's retarded.