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Full Idea
All bodies, as they consist of innumerable parts, are subject to continual changes of their substance. When such changes are gradual, because language could not afford a different name for each state, it retains the same name and is considered the same.
Gist of Idea
We treat slowly changing things as identical for the sake of economy in language
Source
Thomas Reid (Essays on Intellectual Powers 3: Memory [1785], III.Ch 4)
Book Ref
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.112
A Reaction
This is hard to deny. We could hardly rename a child each morning. Simlarly, we can't have a unique name for each leaf on a tree. Economy of language explains a huge amount in philosophy.