INTRODUCTORY LOGIC
LESSON 2 – GENUS AND SPECIES
INTRODUCTION
-
We’re doing categorical logic, which means we’re investigating how we mentally divide the world into
little boxes. All the things that have the same essential features are collected into some group and
described according to a term, which you can imagine as a little “tag” for groups of things. (Imagine
everything in the universe put into a giant pile, and you’re sorting them into individual boxes.)
o ELICT: Where in our daily lives do we see things organized into categories? (Walmart, HEB, school
office, (hopefully) student notebooks, etc.)
o E.g., we look around the world and notice that some things are alive and some things are not.
So, we make a category called “living things” and mentally put all the living things in it.
-
Notice that once we divide the world into categories we can often take a category and divide it even
further:
Living
Things
Plants
Trees
-
Flowers
Animals
...
Mammals
Reptiles
Dogs
Cows
Hamsters
Other stuff
...
Fungi
Bacteria
Now notice that the further we go down the chart the more specific the category labels get. Likewise,
notice how the terms at the top are more general—they are “bigger.” they fit more things in them.
GENUS AND SPECIES
Genus – A category that a term fits into and is more general, broad, or abstract than the original term.
- What is a genus of the term Reptile? [animal, living thing, etc.]
- What is a genus of the term Triangle? [polygon, shape, geometric figure, etc.]
Species – A particular example, or smaller category that fits inside a term’s category.
- What is a species of Triangle? [isosceles, scalene, equilateral, etc.]
- What is a species of dog? [puggle, German shepherd, poodle, etc.]
**Note: Genus and Species in logic are NOT the same thing as genus and species in biology. The idea behind each
is the same, but in biology, genus and species have a very technical meaning. In logic, they are relative terms…A
term can be the genus of one term and the species of another. (e.g., quadrilateral is a species of polygon, but a
genus of squares, trapizoids, etc.)
GENUS AND SPECIES HIERARCHIES
A hierarchy for Triangle:
Triangle
Equilateral
Isosceles
Scalene
Notice two things about it:
1.
2.
We’ve listed every type of triangle that there is. So, our hierarchy is exhaustive.
The species of triangles don’t overlap—every triangle we could ever have will fit into ONLY one of these
categories.
Exhaustive – when every species of a term is listed.
- In real life, it’s usually impossible to list EVERY species. Exhaustive lists are pretty rare.
Mutually exclusive – when the categories are completely separate.
- Every genus and species hierarchy that we draw MUST be mutually exclusive.
Four Rules for Drawing Genus and Species Charts
1.
2.
3.
4.
No overlap! - Species should be mutually exclusive. (No overlap!)
No wrong levels! - Species should be placed on the correct level.
No ambiguity! - Words should be used the same way in the chart.
No parts! Examples only! – Don’t list parts of a term; list kinds or types of that term.
Which rule is being broken in these genus and species charts?
Hot things
Numbers
Even
Fires
Habanero
Peppers
Stoves
Prime
Books
Non-fiction
Animal Farm
Cars
Tires
Door
Engine
Radiator
Instructional
Manuals
Biography
ELICIT some ideas for genus and species charts, and build them with the class. Occasionally suggest erroneous
things—“Would it be okay to put ‘pig’ in this next box? Why not?”