The Cloud of Unknowing is an anonymous work, written by a spiritual director from the Catholic contemplative tradition in the 14th Century. In it, the author describes a type of prayer in which one strives to reject all thought, hoping to experience and be healed by God in stillness of mind. The author calls this state, one in which all thoughts are rejected and the mind is stilled, the cloud of unknowing. He believes that encountering God in this way is the way to a changed spirit and character, and, ultimately, to union with God. The modern Centering Prayer movement is based on the method of prayer found in this book.
Here, the author describes this "contemplative work of the spirit":
"This is what you are to do: lift your heart up to the Lord, with a gentle stirring of love desiring him for his own sake and not for his gifts. Center all your attention and desire on him and let this be the sole concern of your mind and heart. Do all in your power to forget everything else, keeping your thoughts and desires free from involvement with any of God's creatures or their affairs whether in general or particular. Perhaps this will seem like an irresponsible attitude, but I tell you, let them all be; pay no attention to them.
What I am describing here is the contemplative work of the spirit. It is this which gives God the greatest delight. For when you fix your love on him, forgetting all else, the saints and angels rejoice and hasten to assist you in every way – though the devils will rage and ceaselessly conspire to thwart you. Your fellow men are marvelously enriched by this work of yours, even if you may not fully understand how; the souls in purgatory are touched, for their suffering is eased by the effects of this work; and, of course, your own spirit is purified and strengthened by this contemplative work more than by all others put together. Yet for all of this, when God's grace arouses you to enthusiasm, it becomes the lightest sort of work there is and the one most willingly done. Without his grace, however, it is very difficult and almost, I should say, quite beyond you.
And so diligently persevere until you feel the joy in it. For in the beginning it is usual to feel nothing but a kind of darkness about your mind, or as it were, a cloud of unknowing. You will seem to know nothing and to feel nothing except a naked intent toward God in the depths of your being. Try as you might, this darkness and this cloud will remain between you and your God. You will feel frustrated, for your mind will be unable to grasp him, and your heart will not relish the delight of his love. But learn to be at home in this darkness. Return to it as often as you can, letting your spirit cry out to him who you love. For if, in this life, you hope to feel and see God as he is in himself it must be within this darkness and this cloud. But if you strive to fix your love on him forgetting all else, which is the work of contemplation I have urged you to begin, I am confident that God in his goodness will bring you to a deep experience of himself."
– The Cloud of Unknowing, Chapter 3