I've posted before about my dislike of the term 'Intellectual Property'. In particular, I've discussed scarcity - the lack of it in the use of intellectual output. I was reading a few things ("A Great Idea Lives Forever. Shouldn't Its Copyright?", and a reply) recently when I realised that there's another way to look at where copyright comes from, and why property rights should not be applied to creative works. It involves the economic exclusion principle (note: different from the exclusion principle and the Pauli exclusion principle).
Exclusion is the fact that ownership of a piece of property excludes others from owning that same property. Imagine you own a Model T car. Other people can own cars that look just like yours but, crucially, they can't own your one. Ownership is exclusive because physical use is (mostly) exclusive; if I take your car, you no longer have one. Exclusion is where, ultimately, many types of scarcity come from. Some scarcity can be attributed to a lack of natural resources, sure. For secondary goods, however, it can be attributed simply to the fact that one of everything is not enough because one of anything is only enough for one person.
You might see where this leads next, but it's worth emphasising. Non-physical property is different to physical property because of its ease of reproduction. An idea is the simplest example: it is copied instantly in the moment it is taught or conveyed. But we do this point an injustice to stop there. Consider the fact that every person on Earth can have their own copy of every movie, song and book ever produced for a cost that is not out of reach even today; a cost that is inexorably falling. Exclusion does not hold for non-physical property - information can be duplicated without substantial cost. This is what, ultimately, Jefferson meant by those quotes you might read (most notably "he who lights his taper at mine receives light without darkening me"). He wasn't talking about copyright at all, just about the fundamental attributes of creative works.