The Art of Logic · Part 5 · Words and terms · B2–C1
Definition: Genus and Difference
Part 5 of The Art of Logic: a good definition places a thing in its class and then says how it differs from everything else in that class. A bad one can be defeated by a plucked chicken. Defining (a kind of… that…, is defined as, is distinguished from… by), with Philo, Sophia, Marcos and Demetria.
Tonight, the most useful skill in logic: definition.
And there is the problem. A definition must fit the thing, the whole thing, and nothing but the thing.
Following Aristotle, Sister Miriam Joseph teaches that the best definition gives the genus, the wider class a thing belongs to, and the specific difference, the quality that separates it from everything else in that class. A human being is a rational animal: animal is the genus, rational the difference.
The rules: a definition must be convertible with the thing defined (every ship is a vessel built to carry people or goods on water, and everything that is such a vessel is a ship); it must not be circular (“a ship is a shiplike vessel”); it should not be negative when it can be positive; it must not be obscure or figurative (“a ship is the bride of the sea”); and it should state what is essential, not what is…
There are other useful kinds too: a distinctive definition by a property only that thing has; a descriptive one by its features; a causal one by how it is made; and an etymological one by the history of the word: philosophy, the love of wisdom.
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