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 not in the concurrent sensory stream , but within the depths of the <WSD>fourth</WSD> dimension . 
Meaning of the word marked with <WSD> tags:  the fourth coordinate that is required (along with three spatial dimensions) to specify a physical event