<*philosophical terminology*> in the traditional
square of opposition, the relationship between a
universal proposition and its corresponding
particular proposition. Thus, an I is the subaltern
of its A proposition, and an O is the subaltern of its E
proposition. Thus, for example: "Some larks are birds is
subaltern to All larks are birds", and "Some robins are not
fish is subaltern to No robins are fish". Subalternation
is a reliable pattern of inference only on the
assumption of existential import for universal propositions.

[A Dictionary of Philosophical Terms and Names]

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