The question posed was whether or not Themes are inherently derivative of *WordPress*, which they are not. Now, I certainly concede that a Theme could be derivative of another *Theme*. But I would also contend that *almost* all PHP in a template file is non-copyrightable (functions.php is another matter altogether).

As for the line of code you quoted: I don't see anything copyrightable in it. (Trademark might be another matter, if the developer of the Erudite Theme trademarked the Theme name "Erudite".) It's nothing more than a function call, using defined arguments and the defined method of operation for internationalization of text strings.

Most of what is copyrightable in *any* Theme is going to be in the images, CSS, and JS - with perhaps some of the functions defined in functions.php. So, any resemblance between a standard Theme template file (index.php, single.php, archive.php, header.php, footer.php, etc.) is going to consist of non-copyrightable function calls and semantic markup.

(I also concede that any blatant plagiarism - be it from WordPress core or from another Theme - would strengthen (but not guarantee, depending on what was copied) the derivative argument.)

That has absolutely nothing to do with determining whether or not a Theme is derivative of WordPress. Simply put: "dependent" does not equal "derivative".

If you put a Windows .exe on a Linux box (without WINE) and tried to run it, you would likewise get errors. That outcome does not mean that the .exe is *derivative* of Windows.

To be derivative of WordPress, a Theme must incorporate, in some concrete form, actual code from WordPress.

The vast majority of Themes do no such thing. No GPL code is copied from WordPress into a Theme. Themes merely use function calls and the like, which, per applicable copyright case law, is non-copyrightable as "method of operation" content.

I question whether the pool of IP attorneys that you know is a representative sample of all IP attorneys. Besides, unless any of them come out and say that they would take such a case, I don't believe it.

The key point that you are missing is that, according to the Copyright Act and applicable copyright case law, no incorporation has taken place. If the authors of the GPL are saying otherwise, then, yes: I'm saying exactly that they don't know what "derivative work" means, as defined by copyright law. Having heard what Matt Mullenweg (and the SFLC) has to say on the matter, I am convinced that his interpretation of "derivative work" is at odds with copyright law.

All a WordPress Theme incorporates is non-copyrightable function calls and filter/action hooks. Thus, with no incorporation of copyrightable content, a Theme is not inherently derivative.

Show me a Theme distributed combined with WordPress and you might have a valid analogy.

That "one idiot in Florida" is a laywer who does happen to spend part of his practice time on copyright law. That the "advocates" are all in unison is entirely irrelevant. Consensus among the "advocates" does not trump the Copyright Act and applicable copyright case law in defining "derivative work". Further, it's not just (or even primarily) the Galoob case that Wasylik (the "idiot in Florida") references that refutes Matt Mullenweg et al. See also Computer Associates v Altai, Sega v Accolade, and Sony v Connectix.

Certainly. See Sega v Accolade and Galoob v Nintendo. These cases clearly establish that mere linking - and use of internal "methods of operation" in order to provide interoperability are not sufficient to cause a work to be considered derivative.

First, their interpretation that mere linking causes a work to be considered derivative is at odds with applicable copyright case law. Second, note the key assumption in the analysis:

The as if they had been added assumption is critical to their analysis - and has absolutely no bearing on the circumstance of a Theme developer distributing his Theme, wholly separate from WordPress core.

Dependency does not constitute derivative, as per the Copyright Act and applicable case law. For specific reference, see Sega v Accolade and Galoob v Nintendo. In the former, Accolade made Sega Genesis-compatible video games. In the latter, Galoob made the Nintendo-specific Game Genie product. The courts considered neither to be a derivative of the protected work on which they depended.

Again, the GPL doesn't define "derivative work". Rather, the Copyright Act and applicable copyright case law define "derivative work". And the Act and prevailing case law do not consider mere linking to meet the standard for considering a work derivative, which is that a work must incorporate, in some concrete form, copyrightable code from a protected work in order to be considered derivative.

And the PHP-interpreter combination does not take place by the Theme developer, but at run-time, by the end user, who is fully within his rights under the terms of the GPL to cause such combination to take place.

The GPL is a copyright license that derives its validity from the Copyright Act. The GPL limits its scope to distribution of modified or derivative code. Distribution is an activity wholly under the purview of the Copyright Act. Thus, the GPL explicitly states as out-of-scope any activity outside of the Copyright Act, and any activity covered by the Copyright Act not including distribution.

And the "FSF's own interpretation" disagrees with the Copyright Act and applicable copyright case law. Thus, the "FSF's own interpretation" is not legally enforceable.

For reference, see Computer Associates v Altai, Sega v Accolade, Sony v Connectix, and Galoob v Nintendo.

PHP include/require versus exec/system makes no difference whatsoever in determining whether or not a work is derivative of another work. In order to be derivative, a work must incorporate, in some concrete form, copyrightable content of a protected work. (See the Altai decision.)

The GPL is a copyright license. Therefore, it cannot extend the definition of "derivative work" beyond the definition from the Copyright Act and applicable case law. So, if what the GPL states is inconsistent with the Act and case law, it is unenforceable.