Merely by posing the question in that form, the author has already conceded to his opponents the bulk what they themselves have to proof; namely, that "file-sharing" is stealing. By making this concession, the burden now falls upon the author to make a moral justification for "theft."
Not that this burden is insurmountable. Even within traditional approaches to ethics there are weighty moral arguments for "theft" of specific objects from specific persons under specific conditions. Those arguments invariably rest upon conditions of the material or social value of the pilfered item, the proportionate distribution of property between the thief and the victim of theft, the frequency of theft or immediacy of need, and (most importantly) the issue of life or health (e.g., the peasant stealing a sack of wheat from a wealthy nobleman to feed his starving family). This example should illustrate the problem a defender of file-sharing would face availing herself to this exception, given that it would strain the imagination to conceive of a case where downloading music, movies, games, etc. would serve immediate vital needs. To make a moral case for the "stealing" of copyrighted media, the file-sharing advocate really has no recourse to the limit cases of traditional ethics.
Let's be clear that the question “Is stealing music wrong?” is a moral and not a legal (in the sense of "positive law") one. Any group of bureaucrats can, with the fiat of the courts, define what constitutes "stealing." What we are aiming at is the essence of what it means "to steal."
Let's look at "stealing." Granting that there is a synonymous relationship between the verb "to steal" and the noun form "theft" such that we can substitute (mutatis mutandis) one concept for the other, we can avail ourselves to to Merriam-Webster, which defines "theft" as “the act of stealing; specifically: the felonious taking and removing of personal property with intent to deprive the rightful owner of it.” Let's examine what is covered in the "specifically" clause by way of an example:
Jack and Jill live on opposite sides of a hill. Jill has a bucket which she built using scrap materials found on her side of the hill. Jack doesn't have a bucket, or any kind of water-fetching device, lacking the material with which to fashion such a device. Upon learning that Jill has a water-fetching device, Jack goes to Jill's side of the hill, takes her bucket, and leaves.
Turning to the particular wording of our clause, we will set aside the adjective "felonious," since it only establishes the juridical status the “taking and removing”, which we're not really interested in (for the moment, anyway). “Taking and removing” is the act Jack performs in relation to "personal" (belonging to Jill) "property" (the bucket). This act is "intentional," since Jack is purposefully went over the hill to take and remove the bucket, thus "depriving" the "rightful owner" (Jill) of the use of the bucket. The aspect of this scenario which we recognize as wrong is that Jill obviously had a claim on the television set, and Jack unjustly deprived her of ability to make use of that claim.
Now, let's draw our attention to the two words: "removing" and "deprive." Take a second example:
Jill has a free and unlimited supply of fresh running water on her side of the hill. Jack does not, having only a limited supply which, while adequate for his basic needs, requires that he closely monitor his use. Upon learning that Jill, on the other hand, has an unlimited supply of fresh water, Jack takes a bucket (not the one he stole from Jill), goes over the hill, fills his bucket with water, and leaves.In this second example, Jack “takes and removes” water from Jill's side of the hill, but what makes this scenario different is that there is no "depriving" her of the use of that possession. Jill still has free and unlimited access to water. The portion that Jack takes in no way effects her ability to have access to free and unlimited water.
It is unfortunate that such circumstances seldom obtain in nature. Because of the limitations of material objects, most things can only be used by one or a few persons at a given time, exist in limited quantities, or for a limited duration. Those things that have been perceived to be free and of an unlimited supply have been recognized by almost all pre-industrial cultures as available to everyone in the community to use as they see fit, providing their use neither diminishes the same ability for others or in some way imposes limits upon the supply available for the community. This is what has traditionally been referred to in English, "the Commons." The notion of the commons was always applied to material things whose very existence could be jeopardized through its use.
Wouldn't it be great if there were some aspect of reality that did not suffer from these material limitations? That we could somehow have at our disposal, that we could in some way take possession of without depriving others of the same possession? Or that we could reproduce an infinite number of times without diminishing resources upon which it is made?
If Jack approaches Jill and asks for her bucket, Jill has two choices: she can either give him the bucket or not. "To give" means to relinquish your possession of a thing. If Jack & Jill come to an agreement where Jack can use the bucket for a designated period of time, and then he returns it to Jill for a period of time, and back and forth, we usually call this "sharing." But it is only "sharing" in an analogous sense, since at any given time only one of them have possession of the bucket, and the other one doesn't. With physical things, we often use the term "sharing" in this restricted sense: “I am sharing a bench” or “I am sharing my ice cream.” But in both cases what you are doing is relinquishing (giving) a portion of your possession to another.
What makes so-called "intellectual property" different from real property is that it does not suffer from the limitations of materiality described above. What the "intellectual" designates is that it is based on ideas rather than physical matter. Ideas need only a base set of experiences to be duplicated. If Jill shows Jack how to make a bucket, Jack "possesses" the knowledge of making a bucket to the same degree as Jill, without depriving Jill of her own knowledge of bucket-making. Jack might still need some tin to make a bucket, but by being taught by Jill how to make a bucket, he now has the knowledge of bucket-building. The means by which ideas are duplicated is by experience. Anyone who sees, hears, tastes, etc. can potentially duplicate the idea. Ideas can be "given" is such a way that the giver doesn't relinquish possession of what is given. If you have an idea, as soon as you share that idea with others, they have full possession of the idea as well. This is "sharing" in the fullest sense. Additionally, when an idea is shared with others, that idea becomes the building block for new ideas and new applications of that idea.
In an altruistic society (that is to say, a healthy society) the question of ownership would only arise in those circumstances where the materiality of property prevents the equal access by two or more parties. The notion that "theft" would apply to something which two or more parties can have equal access to is not only an absurity, but the questionable party in such a disagreement would be the one who seeks to make a claim of ownership to what could easily be accessible to all.
If, instead of making buckets, Jill composed a song, by hearing & remembering to a sufficient degree to recite the song, Jack now also possesses the knowledge of this song. Jill has a legitimate claim to being the composer of the song, but only way Jill can prevent Jack from reciting the song (if she can't convince him not to) is under the treat of violence. It is only by instituing a system of violence that anyone can effectively convince others of their society to pay for something that is naturally accessible to all.