Some things are "property" prior to any law. The laws protecting such "property" exist to safeguard what is already "property" by custom. Personal property--for example your clothes, the things in your pockets, your car, your house and everything in it--is property of this kind.
Other forms of "property" are entirely creatures of statute. Absent the law defining them as "property", such "property" would not exist. So-called "intellectual property", to the extent that it is "property" at all, is "property" of this kind. It is "property" only because of a deliberate, arbitrary public policy to create such "property".
Most discussions that label copyright infringement as "theft" without any qualification whatsoever seem to ignore this important distinction between property-by-deep-custom and "property"-by-arbitrary-pragmatic-policy. To put the matter more bluntly: those who speak this way are using a kind of lexical bait-and-switch, trying to arouse people's emotional response to theft of tangible personal property and then have it attach to infringement of an arbitrary government-granted monopoly privilege. The things are quite different. To steal personal property is to attack the fundamental order of society. To infringe a government-granted monopoly privilege is to defy society's administrative order. In this respect copyright infringement has more in common with moonshining than it has with theft.
The monopoly privileges are granted for a purpose. The original purpose was to reward the English Crown's favorites with lucrative monopolies and to suppress "seditious" publications: the London Stationer's company became the Crown's censor. The modern purpose of copyright is more benign: "To promote the progress of Science and Useful Arts", that is, to give writers and their publishers a chance to exploit their writings for a time without competition from rival publishers, thereby creating an orderly market for new writings. Once the author has had his chance, though, free market values take over: the work enters the public domain and anyone may exploit it commercially.
The implication here is clear: granting copyrights and patents is a real-world compromise of the free-market philosophy. A freedom which the public had, absent the monopoly, is infringed by the monopoly. (It might even be said that it is the monopolist who is "stealing" from the public.) In theory, though, the monopoly grants are agreed to on all sides. In theory the public, through its representatives, has sacrificed the margin of its freedom in the short term for greater freedom in the long term. If the monopolies encourage new writing, then after the monopolies expire the public domain will be larger than it would otherwise have been. Unfortunately the practice deviates from the theory. Copyright law is not now written with the ultimate goal of enlarging the public domain by encouraging creativity. The laws are written more for the benefit of the moneyed interests than for the benefit of the public.
The foremost problem in modern copyright is not, as the special interests seem to think, widespread infringement by means of digital computer networks. Rather, it is the use of the legislative process by the special interests to prosecute their centuries-long attack on the public domain.
I have no MP3 software, and have never logged on to Napster. I attempt to abide by the copyright law and not to post protected material on the web beyond what I consider to be the boundaries of fair use. But as long as the Money keeps up its brutal attack on the public domain, I don't plan to shed many tears over the Money's difficulties (if they even exist) with Napster and similar peer-to-peer file sharing programs.