Sentence:  In the over-all process , it is difficult to assign a `` pure '' role to each constituent of a built detergent formulation ; and , indeed , there is no more <WSD>reason</WSD> to separate the interrelated roles of the active , builder , antiredeposition agent , etc. than there is to assign individual actions to each of the numerous isomers making up a given commercial organic active . 
Meaning of the word marked with <WSD> tags:  a rational motive for a belief or action