Volumes of polyhedra. Volume

A body is called simple if it can be divided into a finite number of triangular pyramids.
For simple bodies, volume is a positive quantity, the numerical value of which has the following properties:
1. Equal bodies have equal volume.
2. If the body is divided into parts that are simple bodies, then the volume of this body is equal to the sum of the volumes of its parts.
3. The volume of a cube, the edge of which is equal to the unit of length, is equal to the unit of volume.