Nature and concrete

These days I am thinking about the virtual, digital, physical, and material, in addition to the ideas of the original and copy. I never understood why films based on or inspired in a book are demanded some form of fidelity to the latter. That demand is later transferred to the critic, who cannot speak of the cinematic rendition without making a meaningful reference to the preceding incarnation. Why do we think that the film is only an interpretation of the original? I do not denigrate the need to make an allusion as such. Rather, I scratch my head at the insistent demand that the film's integrity be valued and evaluated by having the book as an infallible point of reference.