Sentence:  Obviously , a satisfactory answer to the third question <WSD>is</WSD> imperative , if the argument is to get under way at all , for if there is any possibility of doubt whether the patient 's tactual sensitivity had been impaired by the occipital lesion , any findings whatsoever in regard to the first question become completely ambiguous and fail altogether , of course , as evidence to establish the desired conclusion . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)