CPT symmetry is a fundamental assumption in modern physics; it is in fact a theorem in the quantum theories of relativistic fields. According to the Standard Model of particle physics nature preserves this symmetry. In other words, the laws of physics do not change when all the particles in an interaction are replaced by their antiparticle (C), the three directions in space are reversed (P) and time is reversed (T). It is known that nature violates CP (charge-parity) symmetry, but there is no experimental proof of CPT symmetry violation. The discovery of its violation could undermine Einstein's special theory of relativity.
The CPT theorem was first proved by Wolfgang Pauli, one of the founders of quantum mechanics.