Although or perhaps because both the Enlightenment and its successor Modern projects sought to eschew metaphysics, one of their most characteristic (creations/something that is both produced and producing) copyright relies on a curious but little examined metaphysics.
Copyright, as every law student knows (or should know) does not give anyone the power to exclude others from using ideas or facts. This is, or at least should be, unsurprising. For something to be regarded as a fact it does not need to be “creative” but simply to be factual. ‘Creativity’ with facts is usually regarded as a bad idea. Fraud involves some ‘creativity’ with facts as do many other kinds of dishonesty, which are immoral if not illegal. Ideas, like facts are the common currency of human communication.
Permitting a person to control an idea, and invoking the coercive power of the State to enforce that control is obviously problematic. Why obviously? Attempts to control ideas, whether by public or private parties is censorship, an unwarranted intrusion into freedoms of belief and expression which is central to open societies. There is however another more subtle point.
Its easy if we have an idea that strikes us as important to fondly imagine it to be unique. History teaches us that there is very little which has not been thought, and recorded before. We are not as original as we may often hope to be. Closely related to this is the difficulty which a judge would encounter in determining whether two ideas are the same. If ideas are somehow separate from their expressions* then the difficulty is in determining whether the ideas are the same without resort to their expressions. How can a judge determine that what you mean by your “idea” is the same as the “idea” which I now have, apparently through copying. Are two completely different expressions the same idea in the abstract? Are the ideas identical or just similar? Are there yet other expressions, not yet expressed which could also be covered by the abstract idea? Its readily apparent that asking judges to ascertain if one person has copied another’s ‘idea’ involves judges resolving difficult questions about the nature of consciousness, the relationship between minds and bodies, basically all the big questions in the philosophy of mind. Comparing two expressions of ideas such as two books, while it raises some practical difficulties which require judgment is a lot simpler. For the purposes of judicial economy alone polities chose to grant exclusive rights not to ideas but to expressions of those ideas.
*Of course if someone were to contend that its not really possible to distinguish between ideas and expressions then that person cannot complain when copyright protects only expressions.