Sentence:  Emerson , in his lecture , refers to the `` startling experience which almost every person confesses in daylight , that particular passages of conversation and action have occurred to him in the same order before , whether dreaming or waking , a suspicion that they have been with precisely these persons in precisely this room , and heard precisely this dialogue , at some former hour , they know <WSD>not</WSD> when '' . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words