There are two heinous academic crimes: falsifying results, and plagiarism. The reason why the first is an offence is clear: the whole purpose of research is a search for the truth (whatever that means), and falsified results undermine that purpose. But what is the actual problem with plagiarism? Who, or what, does it hurt? This little book discusses many aspects of the offence.
Posner starts with a "typical" dictionary definition: "literary theft". I prefer the definition: "to pas off another's work as one's own". Plagiarism.org cites Merriam-Webster with both meanings, and concludes that it is fraud: both stealing and lying. But it doesn't feel like the same class of fraud as falsifying results, so, again, why the particular academic outrage?
Posner explores the question by asking, where is the harm? who is the victim?
The first harm identified is to the original author. The plagiarist is setting themselves up in competition, and may do better (and hence the original author do worse) because of having a fraudulently improved product. But what if the original author is long dead? Well, then the second harm is to the plagiarist's other competitors: others striving to fill the same niche, but putting in the honest toil required to do so. This applies both to competing authors, and to competing students, in the case of plagiarised assignments.
I think the "competing author" analysis slightly misses the point. Here it is discussed mostly in terms of market share: your book does better, so you get more money; your publication is cited more, so you get promotion. I think it is more than this, but it is alluded to only in passing by Posner:
By why is it important, even when there is no direct financial benefit (or loss of benefit)? I think it is because academia is (essentially) a form of "gift culture", in the same sense as Open Source is a gift culture. Academic publication is a "gift" to the academic community (particularly when published in open access journals, but even when not, the material is usually readily available to an accredited researcher). In a gift culture, status is related to the value of your gifts (as opposed to the more commonly understood market economy, where status is related to the value of your possessions: hence theft is a crime). This is why attribution is important: plagiarise, and you steal credit for the gift, and hence any status related to that gift. Using precisely the same material, but with attribution, publicises the usefulness gift (unless you are rubbishing the contribution, of course, but in which case you would hardly plagiarise it!), and hence increases the status of the gift. Having been plagiarised myself, I can attest to two conflicting feelings: outrage that someone has stolen my gift, coupled with a (lesser) feeling of flattery that it was thought worth stealing.
Despite passing over this (I think) crucial point in the context of academic plagiarism, there is a lot of interesting analysis in this book. I was particularly interested in the distinction between originality (something completely new; often not what the customer is after) and creativity (which can include improving on an earlier work, and used to be much more common in the past). Without "plagiarism", there would be little Shakespeare, a lot less poetry and music, no West Side Story, or Clueless, or many other "derivative" works that do no explicitly acknowledge their sources. (Presumably, in the past, when this "creative plagiarism" was all the rage, there was less outrage at being copied, since one may very well not have been the originator anyway.)
Posner also carefully distinguishes plagiarism from copyright violation: one can do one and not the other (plagiarise something no longer in copyright; violate copyright with an attributed quotation). Adherence to copyright privileges originality and individualism over creativity; this may not be a good thing. (Again, this is where Open Source wins: no copyright barriers to creativity; but remember to acknowledge!)
There is more good stuff in this quick read: about "reliance" (would people have behaved differently had they known the true author?); about ghostwriters, book-packaging, and celebrity endorsement; about parody and allusion; about textbooks and their necessary lack of originality; about the difference between "writers" and "authors" (who may "authorise" the writing, even if they didn't write it); about deliberate v negligent plagiarism (a distinction not recognised in my own institution for one, who apply the principle of "absolute liability": that it is academic misconduct "regardless of intention"); about the different attitude to plagiarism in different disciplines (it seems to have a very different flavour in the discipline of Law, for example); about whether it should be made a crime or tort. Read this and learn what people get up to, why some get away with it, and why it might not be a bad thing in every circumstance (academia excepted, of course!).