Sentence:  Thus every part and plane of the picture keeps changing place in relative depth with every other part and plane ; and it is as if the only stable relation <WSD>left</WSD> 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:  make a possibility or provide opportunity for; permit to be attainable or cause to remain