This, the clearest of all the classics of mysticism, but with the fuzziest title, is a gem from late 14th century England. The author's name is unknown - not an unfitting circumstance for a mystical writer. The book's principal theme is that God cannot be grasped by the mind but by love alone.

  You will ask me, 'How am I to think of God himself, and what is he?' and I cannot answer you except to say 'I do not know!' for with this question you have brought me into the same darkness, the same cloud of unknowing where I want you to be! For though we through the grace of God can know fully about all other matters, and think about them - yes, even the very works of God himself - yet of God himself can no one think. Therefore I will leave on one side everything I can think, and choose for my love that thing which I cannot think! Why? Because he may well be loved, but not thought. By love he can be caught and held, but by thinking never.  

In their many different idioms the classical spiritual writers have attempted to throw light on the eternal question of union with God. 
Every month we give you a brief passage from a spiritual classic.