In brief, God is inexpressible to the Son.Arius of Alexandria (context here)
For he is in himself what he is, that is, indescribable,
So that the son does not comprehend any of these things or have the understanding to explain them.
For it is impossible for him to fathom the Father, who is by himself.
For the Son himself does not even know his own essence (ousia),
For being Son, his existence is most certainly at the will of the Father.
The Son, as a human, couldn't possibly know his own ousia you see.