Sentence:  Thus every part and plane of the picture keeps changing place in relative depth with every other part and plane ; and it <WSD>is</WSD> as if the only stable relation left among the different parts of the picture is the ambivalent and ambiguous one that each has with the surface . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)