Discrete symmetries as automorphisms of the proper Poincare group

We present the consistent approach to finding the discrete transformations in the representation spaces of the proper Poincar'e group. To this end we use the possibility to establish a correspondence between involutory automorphisms of the proper Poincar'e group and the discrete transformations. As a result, we derive rules of the discrete transformations for arbitrary spin-tensor fields without the use of relativistic wave equations. Besides, we construct explicitly fields carrying representations of the extended Poincar'e group, which includes the discrete transformations as well.