The relevant theorems are:

- There is no TM
**M**such that given any TM and string**u**,**M**can determine whether accepts**u**. - There exists a certain fixed TM such that
no TM
**M**can determine, given an input string**u**, whether accepts w.

Note that these two theorem are * not* interderivable. In the
``Florida story" involving Welty and Shapiro, I idiotically tried
to get the second of these from the first --- and then realized
that was impossible, something I should have remembered from
elementary computability theory (see below)!

Thu Apr 25 14:15:16 EDT 1996