I'm with farfromfearless on this. I don't get writer's block as it stands because I'm never lacking for ideas or for The Next Scene. What stops me, when I am stopped, is exhaustion, or the rest of the world. If you get kicked in the head a couple of times, then it can be the most hellish thing in the world to get writing, even though once you're going it's usually enjoyable and beneficial.
I think that James describes a large body of writers, though (and I'm generalizing) who decide on writer's block and therefore have writer's block. They may want to write, but they're not doing anything to GET there, because they Have Writer's Block.
The important thing to do, when you have writer's block, is to shoot off in a completely random direction and write something you have no use or intentions for.
Yesterday, I was stumped on novel revisions (again: exhaustion) so I stared at it for twenty minutes, and then opened a new document and wrote a short story. Didn't help my novel any, but I'd written something, it was good, and that was that.