A logical ‘definition’, formulated as a biconditional statement, allows you to infer in all cases what is and isn’t in the class defined.

It does not, however, necessarily coincide with the ‘meaning’ of a concept. To use Quine’s example: the groups of “creatures with kidneys” and “creatures with hearts” may 100% overlap, such that if you know a creature has a heart, you know it has a kidney, and vice versa. But that clearly does not mean the two phrases are synonymous.

This is basically just an elucidation of things I’ve written in here in the past.

Nico Chilla