Please, consider the following statements of the book Hamlet:
BARNARDO
“How now, Horatio? You tremble and look pale.
Is not this something more than fantasy?
What think you on ’t?
HORATIO
Before my God, I might not this believe
Without the sensible and true avouch
Of mine own eyes.”
What does the term before my god mean? Is the meaning of it no different from the phrase as god is my witness?