Property is a product of scarcity. If there's only one of something, and we both want it, then we need a rule to determine whose it is. Air is readily abundant, and fungible (one bit of air is much the same as another), so nobody thinks of "owning" it; in a space colony this would not be the case. The common-law definition of "theft" is taking property with the intention of permanently depriving its owner of its use. If taking it does not deprive the owner of anything, then it cannot be theft.
If I write something, I own the piece of paper on which I wrote it. I may sell that piece of paper, or I may keep it locked away in a safe and not let anyone look at it, or I may destroy it. If you take it from me, then I no longer have it, and I can't do what I like with it, and so you are a thief. But if you make a copy of my work, on your own paper, that copy belongs to you. If I were to take it by force, I'd be a thief. As for me, the existence of your copy has not in any way deprived me of the use of mine, so nothing has been stolen from me.
What, then, is intellectual property? It's a metaphor. In the USA, as in most countries, the legislature has decided to grant to artists a limited-term monopoly on profiting from their creation. In principle this is no different than any other government-granted monopoly. If the city grants my company an exclusive license to run taxis, and lets me transfer that right to anyone I like, then I can metaphorically call that right my "property". After all, it behaves in many important ways just like property. But if someone defies my monopoly, and illegally drives a taxi in "my" town, may I properly call him a thief? Of course not. He may be earning fares that I ought to have earned, but the fact is that I did not earn them, and they are not mine. Until the passenger has been conveyed from A to B, the fare belongs to her, and afterwards it belongs to the person who took her there. I have a legitimate claim against the interloper, but theft isn't it, because he has taken no property of mine.
Now, what is the purpose of copyright? Why has the USAn congress chosen to grant it to authors? The constitution gives us the only possible answer. Congress is only authorised to grant copyrights for one purpose: "To promote the Progress of Science and useful Arts". Patents and copyrights give inventors an incentive to invent, writers to write, and artists to art. Devote your time and energy to creating useful and interesting works, Congress says, and if it makes any money within so many years that money will be yours. It follows from this that when considering any possible infringement of copyright, the relevant question to ask is whether, had the artist known that this could legally happen, she might not have bothered to create her work.
In Europe, a new concept has lately become popular – that an artist has "moral rights" in her work. In France, for instance, if I buy a painting, and I decide to destroy it, the artist may prevent me from doing so. I may own the canvas, paint, and frame, but the artist, it is thought, "owns" the artistic way they are arranged. IMHO that is a perverse concept. I do not recognise any such moral principle, and, thankfully, in the USA not only does the law not recognise it, but the law can not recognise it, because the constitution precludes it from doing so.
And now we finally arrive at the Bill Hobbs cartoon reproduced below. Mr Hobbs has deleted it from his web site, and has protested at my putting it up here. I do not recognise the validity of this claim. Mr Hobbs, as the creator, is entitled to any income stream that comes from the cartoon (for the limited but absurdly long period provided by Congress). But he is not, IMHO, entitled to hide it from the world, and prevent anyone from seeing it. The copy that resides on my computer is mine, not his, and the copy that is displayed by your browser is yours, and we may do what we like with it. Since the purpose of copyright is to encourage the creation and dissemination of works of art, using it to suppress a work and prevent its dissemination directly contradicts that purpose, and is therefore not a legitimate use of copyright.
I should also mention Fair Use, since that is an exception Congress has made, even to legitimate uses of copyright. Indeed, it had to make such an exception, since without one copyrights would be incompatible with the first amendment, which of course takes precedence over the original constitution. Fair Use is a fuzzy concept, but it seems to me that Mr Hobbs's cartoon is, in its entirety, an object of public interest and concern, since it's the centre of a news event that has attracted a fair amount of reportage over the past week or so. To understand the news event it's necessary to see the cartoon. My post comments on the cartoon, as well as on Mr Hobbs's attempt to withdraw it, and thus seeing it is essential to the substance of my comment. No mere excerpt or description would suffice. IANAL, but I think this satisfies the Two Live Crew criteria, and is thus legitimately covered by the Fair Use exception.