"... the plays an important role deducing the consequences of symmetry in quantum mechanics. With the tools of group theory many consequences of symmetry are revealed. For example, the s that govern are simply the consequences of rotational symmetry. Quantum mechanics also revealed a new kind of symmetry, that of exchange of identical particles. This lead to a classification of all elementary particles as either s, whose wave function is invariant under interchange of two identical particles, or s, whose wave function changes sign when two identical particles are interchanged. The of such particles is different, with profound implications for their behavior in aggregate."