Sentence:  Often it is recognized that all the details of the pattern may not be essential to the outcome but , because the pattern was empirically determined and not developed through theoretical understanding , one <WSD>is</WSD> never quite certain which behavior elements are effective , and the whole pattern becomes ritualized . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)