How do novels, plays, or works of music exist?

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How do novels, plays, or works of music exist? Consider the Iliad. The original copy of the Iliad was lost long, long ago, yet the Iliad continues to exist through its copies. If all original-language versions of the Iliad were to disappear, leaving only translations, one would assume the Iliad would continue to exist. What if all copies of the Iliad in any language and in any material form were destroyed, and we were left with nothing but the memory of the Iliad? Would it then cease to exist, until someone (presumably with photographic memory) decided to write it down again? What if all memory and knowledge of the Iliad were erased, but copies still existed, lying around in old boxes where nobody remembered them? Would it still exist if this were the case? How can we conceptualize the existence of things, like an ancient epic poem, which exist in physical form yet are not dependent on these forms?