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As I have already given you to understand: — Something that even those who know it cannot say. Falk is referring to the "secret" of Freemasonry, suggesting it is an internal experience or insight that defies verbal description.
So, an absurdity. The original German "Unding" literally translates to a "non-thing." In this philosophical context, Ernst is arguing that if something cannot be expressed, it is a logical impossibility or nonsense.
Do not be so hasty.
Whatever I have a clear concept of, I am also able to express in words.
Not always; and often at least not in such a way that others, through those words, receive exactly The sentence ends abruptly here due to the page break, following the German "vollkommen eben." The catchword "den-" suggests the next word is "denselben" (the same).