 The discrete symmetries C, P and T are connected by the so-called CPT theorem, first proved by Lüders and Pauli in the early 1950s, which states that the combination of C, P, and T is a general symmetry of physical laws. For philosophical reflections on the meaning and grounding of the CPT theorem see Wallace (2009) and Greaves (2010). A discussion of the proofs of the theorem, both within the standard quantum field theory framework and the axiomatic field theory framework, is provided in Greaves and Thomas (2014).