Nature isn't on JSTOR? Really? D: That's suck. They have it in their for-pay online archives at nature.com, too.

I found a for-pay site offering the de Witt paper, too, (it's also on de Witt's CV); they're both real. That one might be harder to find, though, it looks like it's a conference paper that was only published in the conference anthology. (Alas, all the others in the bibliography exist only in an alternate timeline.) If I wasn't so broke I might just man up and buy them.