The Art of Logic · Part 6 · Words and terms · B2–C1
Division: One Rule at a Time
Part 6 of The Art of Logic: to divide a class well, use one rule at a time, leave nothing out, and let nothing fall into two groups at once. Break the rules and you get an imaginary encyclopedia that lists animals “belonging to the emperor”. Classifying groups (those that… and those that…, the former, the latter), with Philo, Sophia, Marcos and Demetria.
Definition puts a thing into its class. Division…?
Takes a class apart into its kinds. Animals into those with blood and those without; scrolls into Greek, Latin, Hebrew and Egyptian.
Logical division separates a class (a genus) into its kinds (its species) according to one basis: one rule. Sister Miriam Joseph gives the rules: use only one basis at each step; the kinds must be mutually exclusive, so that nothing falls into two of them; they must be exhaustive, so that nothing is left out; and the division should go step by step, without skipping levels.
The safest division is a dichotomy: dividing a class into what has a quality and what does not, animals with backbones and animals without. It is always exclusive and exhaustive, though not always informative.
Logical division is not the same as partition, cutting a whole into its parts. A ship is partitioned into hull, mast and sail; but “ship” is divided into warships, merchant ships and fishing ships. A hull is not a kind of ship.
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