Once you know genus and species, you can sort every true statement "Every S is P" by how the predicate belongs to the subject. Since Porphyry wrote about them in the third century, logicians have counted five ways, called the predicables.
| Predicable | How it belongs | Example for "triangle" |
|---|---|---|
| Genus | the wider class it belongs to | is a polygon |
| Specific difference | what sets it apart within the genus | is three-sided |
| Species (definition) | genus + difference: the whole essence | is a three-sided polygon |
| Property | not in the definition, but follows from it, so every member has it | has angles that add up to 180° |
| Accident | may or may not belong; the thing would be the same kind without it | is drawn in red ink |
Key idea
Genus, difference and species belong to what a thing is. A property is not part of the definition but always comes with it. An accident could be missing without the thing changing its kind.
Why it works
The test for each is a question. Is it the wider class? (genus) Is it what marks this kind off from others in that class? (difference) Is it both together? (species) If it is none of those, could a member of the kind lack it? If no member could lack it, because it follows from what the thing is, it is a property. If a member could lack it and still be the same kind of thing, it is an accident.
Worked examples
Worked example: A property
Subject: square. Predicate: "has diagonals that cross at right angles". Which predicable?
- It is not the genus (rectangle), not the difference (four equal sides), and not the full definition.
- Could a square lack it? No: every square has such diagonals, because of what a square is.
- So it is a property.
Answer: property
Worked example: An accident
Subject: rectangle. Predicate: "is 4 cm long". Which predicable?
- It is not part of the definition, and it does not follow from it.
- A rectangle 5 cm long is just as much a rectangle. So it can be missing: an accident.
Answer: accident
Common mistake
Confusing property and accident. Both lie outside the definition, so they look alike. The test: a property belongs to every member because of what the thing is (every triangle's angles add up to 180°); an accident does not (this triangle happens to be red). Ask "could one of them lack it?"