It is in spirit and in mind, then, that he says that one should pray and sing to God; he does not say anything at all about the tongue. The reason is… - John of Apamea

For God is silence, and in silence is he sung by means of that psalmody which is worthy of Him. I am not speaking of the silence of the tongue, for if someone merely keeps his tongue silent, without knowing how to sing in mind and spirit, then he is simply unoccupied and becomes filled with evil thoughts: he is just keeping an exterior silence and he does not know how to sing in an interior way, seeing that the tongue of his ‘hidden man’ has not yet learnt to stretch itself out even to babble. You should look on the spiritual infant that is within you in the same way as you do on an ordinary child or infant: just as the tongue placed in an infant’s mouth is still because it does not yet know speech or the right movements for speaking, so it is with that interior tongue of the mind; it will be still from all speech and from all thought: it will simply be placed there, ready to learn the first babblings of spiritual utterance.

Thus there is a silence of the tongue, there is a silence of the whole body, there is the silence of the soul, there is the silence of the mind, and there is the silence of the spirit. The silence of the tongue is merely when it is not incited to evil speech; the silence of the entire body is when all its senses are unoccupied; the silence of the soul is when there are no ugly thoughts bursting forth within it; the silence of the mind is when it is not reflecting on any harmful knowledge or wisdom; the silence of the spirit is when the mind ceases even from stirrings caused by created spiritual beings and all its movements are stirred solely by Being, at the wondrous awe of the silence which surrounds Being.

Do not imagine, brother, that prayer consists solely of words, or that it can be learnt by means of words. No, the truth of the matter, you should understand, is that spiritual prayer does not reach fullness as a result of either learning or the repetition of words. For it is not to a man that you are praying, before whom you can repeat a well-composed speech: it is to Him who is Spirit that you are directing the movements of your prayer. You should pray therefore in spirit, seeing that He is Spirit.