Full Idea
We must acknowledge that organic bodies as well as others remain 'the same' only in appearance, and not strictly speaking. It is rather like the river whose water is continually changing, or like Theseus's ship which Athenians constantly repaired.
Gist of Idea
Bodies, like Theseus's ship, are only the same in appearance, and never strictly the same
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.27)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.231
A Reaction
This is Leibniz's standard view, that something only remains the same if it has a unifying substance, and so a collection of planks is just an aggregate, and doesn't have any identity to begin with.