I am basically listing out all topics in science (natural science) and was looking at branches of physics. Wikipedia says that physics can broadly be classified into following: - Classical Mechanics, Relativity, Quantum Theory, Thermodynamics and Electromagnetism. On what basis is this done? Can we classify physics on the basis of what are we studying. E.g. how particles move is Mechanics, what makes them move is Electromagnetism and gravity, what happens at microscopic level is Quantum Theory ... something like this.