EXCLUSIVE

Spike Lee is breaking a sacred rule by criticizing "Django Unchained" for its pervasive use of the n-word -- so says Sarah Silverman ... who tells TMZ, art is art ... and "you can't tell art what to do."

Sarah was out in L.A. when we asked about Spike's refusal to watch the new Quentin Tarantino flick, which features the n-word roughly 110 times. Spike criticized the film, claiming it mocks slavery and is "disrespectful to my ancestors."

But last night, Sarah shot back, "Doesn't it take place like during slavery? Wouldn't it be odd if they didn't have that horrific word in it?"

Sarah -- a self-professed Spike Lee fan -- said, "[Spike's] got a lot of mishegas with a lot of art. I think you can't really tell art what to do."

Or can you ...