On being transformed into Greta Garbo cell by cell

 Suppose that a scientist were to begin replacing your cells, one by one, with those of Greta Garbo at the age of thirty. At the beginning of the experiment, the recipient of the cells would clearly be you, and at the end it would clearly be Garbo, but what about in the middle? 

It seems implausible to suggest that you could draw a line between the two—that any single cell could make all the difference between you and not-you. 

There is, then, no answer to the question of whether or not the person is you, and yet there is no mystery involved—we know what happened. 

A self, it seems, is not all or nothing but the sort of thing that there can be more of or less of.  When does a person, descending into dementia or coma, cease to be? There is no simple answer—it is a matter of degrees