|Is, philosophy: the word “is” has different functions like expressing an identity e.g. A is B, attributing a property e.g. the sky is blue or being contained in a set e.g. "Heaven" is a concept. These functions must not be confused.|
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A) Identity: e.g. "Cicero is Tullius"
B) Predication: e.g. "Cicero is a Roman".
Stabilization function: The difference consists in the different stabilization functions of the lowest type of "is" not in a different lexicon sense.
"Is"/identity/function/Millikan: the function of the "is" in identity statements is parasitic on the representative-referring stabilization functions of the flanking terms.
Suppose there are certain descriptions on both sides: then their referential function is only one of two alternating stabilization functions:
Referential/descriptive: although certain descriptions always have a referential function, they can also be descriptive at the same time.
N.B.: then the "is" of identity is never just a pure "is" of identity. It is always also an "is" of predication.
"Is"/Identity/Predication/Millikan: identity and predication are not separate families or separate lowest types.
R. G. Millikan
Language, Thought, and Other Biological Categories: New Foundations for Realism Cambridge 1987