ABSTRACT
A physical theory aims to give a description of the properties and behaviour of a physical system, both qualitatively and quantitatively. Qualitatively a theory should explain the behaviour of a given physical system in terms of some fundamental properties of the system. The study of the mathematical aspects of theoretical physics is often referred to as mathematical physics. Apart from the requirement of mathematical consistency, a theory is judged by how well its predictions agree with empirical observations, especially by its ability to make new and experimentally verifiable predictions. A physical system possesses many physical quantities which reflect the properties of the system. A physical quantity is a property of the system which can be physically observed and measured to yield numerical values. To emphasise their measurability, these quantities are called observables. The relationship between states and observables can be complicated, depending on whether we are dealing with classical systems or quantum systems.
