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.
Chapter 12. On the figures of geometry.
1. The first of the figures on a plane surface is the circle, a figure that is plane, and has a circumference, in the middle of which is a point upon which everything converges (cuncta convergunt) which geometers call the center, and the Latins call the point of the circle.
2. A quadrilateral figure is one on a plane surface, and it is contained by four straight lines....
3. A sphere is a figure of rounded form equal in all its parts.
A cube is a solid figure which is contained by length, breadth, and thickness.
5. A cone (conon) is a solid figure which narrows from a broad base like the right-angled triangle.
6. A pyramid is a solid figure which narrows to a point from a broad base like fire. For fire in Greek is called πῦρ.
7. Just as all number is contained within ten so the outline of every figure is contained within the circle.