Licensing is a big deal in the software and cultural freedom movements; there are a lot of licenses available in both domains (probably too many), and people have strong opinions about what licenses and license components are better or worse. But in the truly libre category of licenses, maybe the most controversial aspect of licensing is that of copyleft, a powerful copyright hack that uses copyright itself in a sort of judo move to force those to make derivatives to give their contributions back to the commons.

There are two primary copyleft licenses, the GNU GPL for software (and some other categories of functional) works (and the related AGPL and LGPL) and CC BY-SA for non-software (generally cultural) works. But I don't intend to go into details on copyleft or the licenses themselves, there's plenty of resources about that already on the internet.

What I'm more interested in exploring here is the perspectives on copyleft. Is copyleft good? Is it bad? A lot of people have extremely strong opinions about it. Actually that's an understatement; if digital ink were made manifest, the amount spilled over copyleft could fill at least one olympic sized swimming pool. But despite all the heated debates about copyleft, I've never really found a good breakdown about what those arguments are. I actually think it's not too hard to separate the arguments categorically, so here's my attempt to do so.

Even though I'm on the overall-in-support side of things (I am actually conditionally in strategic support of copyleft and think the decision about whether to use copyleft or not should be weighed on a case by case basis; more about that at the end) I'm going to start by discussing the objections before I move to the support side. Generally I think the objection side of things is a bit trickier (and intellectually, maybe a bit more interesting to analyze) than the support side, so I'll go to that first before I explain why one might actually find copyleft to be a valuable tool. (A slight amount more caveat: I'm not claiming to not have bias here; I do. But again, I'm not completely on one side or the other, and I think the decision about whether to apply copyleft to your project is best made by understanding both the pros and the cons.)