Full Idea
The memory criterion for personal identity permits 'branching' (where two things can later meet the criteria of persistence of a single earlier thing), which presents it with serious problems.
Gist of Idea
The memory criterion has a problem when one thing branches into two things
Source
report of Bernard Williams (Personal Identity and Individuation [1956]) by Cynthia Macdonald - Varieties of Things Ch.4
Book Reference
Macdonald,Cynthia: 'Varieties of Things' [Blackwell 2005], p.144
A Reaction
Of course, any notion of personal identity would have serious problem if people could branch into two, like fissioning amoeba. If that happened, we probably wouldn't have had a strong notion of personal identity in the first place. See Parfit.