  It is well known that Sullivan showed that the mapping class group of a
simply connected high-dimensional manifold is commensurable with an arithmetic
group, but the meaning of "commensurable" in this statement seems to be less
well known. We explain why this result fails with the now standard definition
of commensurability by exhibiting a manifold whose mapping class group is not
residually finite. We do not suggest any problem with Sullivan's result: rather
we provide a gloss for it.
