The spine
Fields
Seven areas of mathematics, each with its own way of arguing. An essay belongs to exactly one of them and to any number of themes.
Geometry
Shapes, and the arguments you can make by rearranging them.
Analysis
Limits, curves, and what happens when you keep going.
Algebra
Structure: what stays true when you change the numbers.
Discrete
Counting, graphs, and things that come in whole pieces.
Topology
What survives bending, and what does not.
Probability
Randomness with a shape.
Number
The whole numbers, and how much structure they turn out to have.
A field says what an essay is about. Themes say what it rhymes with, ladders say how deep it goes, and the concept index says which essays name the same objects.