One may peruse an encyclopedia of physics without ever encountering the word "matter", even though the entire work deals with nothing else. For example, solid state physicists study material things in the solid state, not the latter in itself. A major reason for the omission of the noun "matter" and the adjective "material" is that the concept of matter is a very general ontological category: in physics there is no general theory of matter, just as there is no general theory of processes.