Sentence:  It resembles , too , pictures such as Du^rer and Bruegel did , in which all that looks at first to <WSD>be</WSD> solely pictorial proves on inspection to be also literary , the representation of a proverb , for example , or a deadly sin . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)