Sentence:  Nevertheless , there are notably frequent instances of deja vu , in which our recognition of an entirely novel event <WSD>is</WSD> a feeling of having lived through it before , a feeling which , though vague , withstands the verbal barrage from the most impressive corps of psychologists . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)