chapter  2
The Concept of Complete Structure
Pages 4

Thus, for example, the atom is the constitutive unit of all physical objects, whether we are talking about a star, a planet, a gaseous nebula or smaller objects such as rocks or molecules. Space, time or force fields are significant entities in physics but they are not generally called physical objects. These can be located and are made up of something "material". Therefore, the atom is a particular case of a complete structure.