I think that's it:
http://en.wikipedia.org/wiki/Reductio_ad_absurdum. Thank you!
This led to this
very useful page, which makes me wish "Contra principia negantem disputari non potest" would fit as a user tile. Alas, my signature is otherwise engaged.
Whoops, further reading found exactly what I was after, recognized as soon as I saw it: "cum hoc ergo propter hoc", or "correlation does not imply causation". The increase in squirrel population and lack of visible cats does not mean the squirrels ate the cats.
Wait, wait! Edit number 5 on the post. What I was
really after was: Post hoc ergo propter hoc.