(A briefer argument of what was argued below.)
To say “nothing is self-evident” is to deny that any word means something, or that anything can be known to be in a genus. A short experiment:
1.) say a word (x).
2.) say what it means, or give a genus of the word (y).
3.) now ask “why is x a y?”
The answer is “because that’s what ‘x’ means! Don’t you remember why you said it?”
This is what we mean when we say “something is known through itself”. It is statements like this that we say are self evident, and which form the basis of what the ancients called scientia or episteme: certain knowledge through causes.
Just Thomism 