You must know that weather or not you are practicing mental prayer has nothing to do with keeping your lips closed. If, while I am speaking with God, I am fully conscious of doing so, and if this is more real to me than the words I am uttering, then I am combining mental and vocal prayer. I am amazed when people tell me that you are speaking with God by reciting the Paternoster even while you are thinking of worldly things. When you speak with a Lord so great, you should think of Who it is you are addressing and what you yourself are, if only that you may speak to Him with proper respect. How can you address a king with the reverence he deserves unless you are clearly conscious of his position and yours?
