 Today there are two rather different groups of theories that may be described as falling under the term logic of action. One, the result of the creation of Nuel Belnap and his many collaborators, may be called stit theory (a term that will be explained in the next paragraph). The other is dynamic logic. Both are connected with modal logic, but in different ways. Stit theory grew out of the philosophical tradition of modal logic. Dynamic logic, on the other hand, was invented by computer scientists in order to analyse computer action; only after the fact was it realized that it could be viewed as modal logic of a very general kind. One important difference between the two is that (for the most part) actions are not directly studied in stit theory: the ontology does not (usually) recognize a category of actions or events. But dynamic logic does. Among philosophers such ontological permissiveness has been unusual. Hector-Neri Castañeda, with his distinction between propositions and practitions, provides one notable exception.