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Full Idea
So-called ostensive definitions need not literally involve ostension, e.g. pointing, but they must involve genuine reference of some sort (in this case reference to a sample of water).
Gist of Idea
Ostensive definitions needn't involve pointing, but must refer to something specific
Source
Nathan Salmon (Reference and Essence (1st edn) [1981], 4.11.2)
Book Ref
Salmon,Nathan: 'Reference and Essence (2nd ed)' [Prometheus 2005], p.101