If you pray with your lips but your mind wanders, how do you benefit? ‘When one buildeth, and another pulleth down, what profit have they then but labour?’ As you labour with your body, so you must labour with your intellect, lest you appear righteous in the body while your heart is filled with every form of injustice and impurity. St. Paul confirms this when he says that if he prays with his tongue—that is, with his lips—his spirit or his voice prays, but his intellect is unproductive: ‘I will pray with the spirit, and I will pray also with the understanding.’ And he adds, ‘…I wish to speak five words with my understanding,…than myriads of words in a tongue.’
Gregory of Sinai
