Chen Ning Yang Discrete Quotations

The law of right-left symmetry was used in classical physics but was not of any great practical importance there. One reason for this derives from the fact that right-left symmetry is a discrete symmetry, unlike rotational symmetry, which is continuous.

The three discrete invariances - reflection invariance, charge conjugation invariance, and time reversal invariance - are connected by an important theorem called the CPT theorem.