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Full Idea
Continuity is not a sufficient criterion of material identity; it is sufficient in many cases, such as rocks and tables, where the appearances change slowly, but in others, such as the parts of an approximately homogeneous fluid, it fails us utterly.
Clarification
A 'homogeneous' has the same texture throughout, such as water
Gist of Idea
Continuity is a sufficient criterion for the identity of a rock, but not for part of a smooth fluid
Source
Bertrand Russell (The Relation of Sense-Data to Physics [1914], §XI)
Book Ref
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.164
A Reaction
It might be debatable to what extent the 'parts' of a homogeneous fluid have identity. How many 'parts' are there in a glass of water? This seems, now, a problem for internalists; externalists can define the identity by the unseen molecules.