You Got a License for that Thing?

Hello everyone! First an introduction and disclaimer: I have no credentials yet as an author of open textbooks of the sort that most people here are writing, nor have I yet even used one of them. But I was one of the principal authors of the homotopy type theory book, which is sort of a cross between a research monograph and a graduate textbook, and was developed as a group project on github and released freely.

In this post, I'd like to start a conversation about licensing, beginning with an overview for those who may be new to the options. With good reason, most open textbooks seem to be published under a Creative Commons license. The most permissive CC license is CC BY, which basically allows others to do as they wish with a work as long as they give credit to the original author. The other CC licenses add additional restrictions, which can be chosen from the following:

• ShareAlike (SA), which requires anyone who modifies a work to release the modified version under the same license terms. This is roughly comparable to the "copyleft" provisions in open-source licenses such as the GNU GPL.
• NoDerivs (ND), which allows redistribution of a work (with credit) but prohibits any modifications.
• NonCommercial (NC), which prohibits use of a work or its derivatives for commercial purposes.

Of course, it doesn't make sense to combine SA with ND, but other than that, any combination of the above is available.

Now when choosing between the available licenses, of course we ought to think about our goals. Here are some that seem relevant to open textbooks:

• We want our textbooks to be available for free (as in beer). This can be accomplished with any CC license.
• We want to get credit for our work. This can also be accomplished with any CC license, since they all include BY.
• We want to avoid the need to reinvent the wheel. E.g. it's ridiculous for every author of a new calculus textbook to have to come up with all new exercises since all exercises in existing books are under copyright. This argues against the ND clause: we should allow future textbook authors to incorporate our work into theirs. I haven't seen any open textbooks that use ND, so there seems to be fairly wide consensus on this point.
• We want to break the grip of expensive conventional publishers on the textbook market. Making good textbooks available for free, and avoiding wheel-reinvention, are good starts on this. But an additional worry we might have is that since we're making our work available for free, a commercial publisher might take what we've done, modify and "improve" it a bit, and then sell the result at an exorbitant price. How can we prevent that with a license? We could choose a NC license, preventing anyone from using our work for commercial purposes. Or we could choose a SA license, ensuring that anyone who modifies our work must also make their modified version freely available. Or we could combine the two.
• We want to spread the philosophy of openness. This argues for SA: we should ask anyone who modifies our work to also join the "open" community.

These goals seem to argue for using either BY-SA or BY-SA-NC. Now, perhaps surprisingly, these two licenses are completely incompatible with each other. If Alice licenses her book under BY-SA, then any derived work must also be licensed under BY-SA; and in particular, it cannot be licensed under BY-SA-NC. This makes sense: Alice doesn't want derived works to add more restrictions on the use of the material than she did, and a prohibition on commercial use is an additional restriction. Similarly, if Bob licenses his book under BY-SA-NC, then any derived work must also be licensed under BY-SA-NC; and in particular, it cannot be licensed under BY-SA. This also makes sense: Bob doesn't want his work to be used for commercial purposes, so he doesn't want any derived work to be used for commercial purposes either, whereas BY-SA would permit that.

However, this means that if Eve wants to create a derived work incorporating elements from both Alice's book and Bob's book, she is stuck: there is no license that she can give to it. Therefore, in the interests of interoperability and avoiding reinvention of the wheel, it might be in the interests of the open math book community to establish conventions about which of these licenses is preferable. In other words, should commercial use of open math books be allowed?

I've noticed that a number of prominent open textbooks use BY-SA-NC. However, there are good arguments to be made in favor of BY-SA instead. The web site Freedom Defined has a very detailed writeup, which I recommend reading. I'll just mention briefly a few of their points:

• The basic free software licenses, such as the GPL, permit commercial use, and for good reasons.
• Because BY-SA-NC is incompatible with BY-SA, it makes your work incompatible with anything that uses the latter license. For instance, you can't incorporate content from Wikipedia.
• SA alone is sufficient to prevent exploitation of our work by evil publishers, since anyone who makes a derived version and sells it must also make it freely available under BY-SA.
• NC prevents beneficial commercial uses. For instance, if someone in the developing world wants to print copies of our textbook and sell them at a small profit to people without Internet access, NC prevents it.

I do recommend reading the whole thing. Then let's discuss in the comments. Is there a good reason for choosing NC?

Michael Shulman
Department of Mathematics and Computer Science
University of San Diego