Sean,
Although I wanted to be one, I'm not a copyright lawyer (I got pulled into tax law instead). So my explanation is based on a copyright law class 11 years ago in law school. That said, copyright law, in the US, protects "original works of authorship," fixed in a tangible medium including literary, dramatic, musical, artistic, and other intellectual works.
Ideas themselves are not protected. According to the Copyright Act of 1976, in no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
For example, a paper describing a political theory is copyrightable. The paper is the expression of the author's ideas about the political theory. But the theory itself is just an idea, and is not copyrightable. Another author is free to describe the same theory in their own words without infringing on the original author's copyright.
An artist's particular sculpture is protected by copyright. What's not protected is the right to use a particular material or subject matter. In music, copyright protection extends to the publication of the song or musical arrangement and the sound recording. The use of a particular note or instrument to produce a particular sound is not protected because that's an idea. But how the musician arranges the notes, timing, lyrics, etc. are protected. The copyright protects the authorship of some intellectual "thing".
Of course, like all areas of law, there are limitations, exceptions, etc. for permitted uses of copyrighted works.
Now I'll wait for a copyright lawyer to correct me...