1. The four-fold division of geometry is into plane figures,
numerical magnitude, rational magnitude, and solid figures.
2. Plane figures are those which are contained by length and breadth.
Numerical magnitude is that which can be divided by the numbers of
arithmetic.
3. Rational magnitudes are those whose measures we can know, and
irrational, those the amount of whose measurement is not known.
4. Solid figures are those that are contained by length, breadth, and
thickness, which are five in number, according to Plato.