It is worth knowing, then, that the appetite is the mouth of the will. It is opened wide when it is not encumbered or occupied with any mouthful of pleasure. When the appetite is centered on something, it becomes narrow by this very fact, since outside of God everything is narrow. That the soul have success in journeying to God and being joined to him, it must have the mouth of its will opened only to God himself, empty and dispossessed of every morsel of appetite, so God may fill it with his love and sweetness; and it must remain with this hunger and thirst for God alone, without desiring to be satisfied by any other thing, since here be low it cannot enjoy God as he is in himself. And what is enjoyable—if there is a desire for it, as I say—impedes this union. Isaiah taught this when he said: All you who thirst, come to the waters (Is. 55:1). He invites to the abundance of the divine waters of union with God only those who thirst for God alone and who have no money, that is, appetites.
Saint John of the Cross
Letter 13 to a Discalced Carmelite friar
Segovia, 14 April 1589
John of the Cross, St. 1991, The Collected Works of St. John of the Cross, Revised Edition, translated from the Spanish by Kavanaugh, K and Rodriguez, O with revisions and introductions by Kavanaugh, K, ICS Publications, Washington DC.