Sentence:  A `` mental image '' subconsciously impressing us from beneath its language symbols in wakeful thought , or consciously in light sleep , <WSD>is</WSD> actually not an image at all but is comprised of realities , viewed not in the concurrent sensory stream , but within the depths of the fourth dimension . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)