<logic, philosophy of language> the reciprocal relationship between two categorical propositions of opposite quality such that the predicate term of each is the complement of the predicate term of the other. Obversion is a valid immediate inference for categorical propositions of every form. Thus, for example: All lizards are reptiles and No lizards are non-reptiles. No spiders are insects and All spiders are non-insects. Some fish are birds and Some fish are not non-birds. Some mammals are not dogs and Some mammals are non-dogs. They are all legitimate cases of obversion; either member of each pair can be substituted for the other.
[A Dictionary of Philosophical Terms and Names]
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