How about a definition like - "If I create something, I own it and have the right to profit from it." Why should my right to profit from what I create have a limited term? Why should it be ok for someone to copy it just because they have the ability to?
Copyright prevents the free speech of others. There's some justification for it doing so. But it's not a natural, or absolute, right.
Also, satire isn't protected, but parody is.
Edit: copyright should be more limited than physical property rights, so I'm in agreement. But neither can be absolute when the government only has a piece of paper to abide by that prevents them from being revoked.
Copyright is only a government-granted, temporary monopoly on that idea to incentivize you to live from creating them in the first place and to create more.
Once you have given your idea to someone else, where does your natural right come from to control what they do with it?
This is a much broader question about the rationale behind copyright law and the concept of intellectual property. It is really beyond the scope of the question posed (is copyright violation theft?).
You are not born in isolation from the rest of us. Everything you create originated from an idea that you learned from somebody else. You have no right of ownership over something that is not entirely yours.
It doesn't seem to be inherently an argument against the entire idea of "personal property", it seems to be inherently an argument against the idea that property (personal, real, or otherwise) is a matter of absolute right, rather than a matter of an exchange in which owners are granted privilege at the expense of restrictions on everyone else's freedom because of a perceived social benefit to that exchange -- where the parameters, then, of the particular privileges of owners become subject to analysis of whether the exchange really does have net social value.
In regards to intellectual property of the types dependent, in US law, on the Constitution's so-called "Copyright Clause", this is pretty explicitly the premise in the US Constitutional system; in addition, the differences in the privileges and limitations of real and personal property rights (and the particular parameters of particular subclasses of each), as well as things like eminent domain, are very hard to conceptualize under a model of property-as-absolute-right unless you simply define each feature as its own axiom of property rights, except the ones you don't like and define them as violations of axiomatic rights, whereas the whole regime of property rights and its evolution makes a lot more sense under a social benefit exchange view with an evolving view of what provisions actually provide social benefit.
What if you spend a year making a beautiful cabinet? You didn't invent the tree, or the saw, so it's okay if I come by and cart the cabinet over to my house, right?
Note that I agree that you should (or, rather, that as a society we should make sure that it's possible under reasonable conditions), but my reason for agreement is not able to justify perpetual copyright.
My reasoning happens to be similar to what the U.S. Constitution says, but I can imagine others having different reasoning for the same idea.
He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper [candle] at mine, receives light without darkening me. That ideas should freely spread from one to another over the globe, for the moral and mutual instruction of man, and improvement of his condition, seems to have been peculiarly and benevolently designed by nature.... Inventions then cannot, in nature, be a subject of property.