Variable Set Theory?

Mike Barr posted in the cats mailing list the following:
Around 1986, Colin McLarty, Charles Wells, and I noticed that Scientific American had recently published a few articles on subjects mathematical
and decided to try to write one on variable set theory, AKA topos theory.
Unfortunately, that particular window at Sci Am quickly closed.  Our article was turned down and we forgot about it.  More recently, I decided it might be interesting, or at least amusing to throw it open to the
categorical community.  The result can be found at
ftp://ftp.math.mcgill.ca/barr/pdffiles/vst.pdf

(the picture is apparently Grothendieck lifting Michael Atiyah, from facebook, so who knows if it's real...)


Further news: Since the ftp site in Montreal is having some issues at the moment, Mike Barr was kind enough to send us a copy of the paper, which I'm posting here. Thanks Mike!