Sentence:  A `` mental image '' subconsciously impressing us from beneath its language symbols in wakeful thought , or consciously in light sleep , is actually not an image at all but is comprised of realities , viewed <WSD>not</WSD> in the concurrent sensory stream , but within the depths of the fourth dimension . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words