The reality of atoms can be deduced from the reality of the devices by which the atoms are prepared and registered. A new, most general definition of the concept of “physical object” is given. The objects must not be classical; nevertheless they can be described objectively. Atoms are not such objects.

There are various concepts of states in physics. Sometimes they are not formulated precisely. This leads to many misunderstandings. We try to give rigorous definitions for classical states, statistical states, and hidden states and to describe the relations between these various concepts.