  If the universe expands exponentially without end, ``ordinary observers''
like ourselves may be vastly outnumbered by ``Boltzmann's brains,'' transient
observers who briefly flicker into existence as a result of quantum or thermal
fluctuations. One might then wonder why we are so atypical. I show that tiny
changes in physics--for instance, extremely slow variations of fundamental
constants--can drastically change this result, and argue that one should be
wary of conclusions that rely on exact knowledge of the laws of physics in the
very distant future.
