 An action is obligable if and only if it is, in a certain sense, substitutable. There are several different senses of ‘could’ and hence of substitutability. Initially Broad focuses on a sense of ‘could’ which he calls voluntary substitutability. Consider an action A that was not performed. Suppose that if the agent had willed (chosen, decided) to do A she would have done it. In that case A is voluntarily substitutable. Suppose, however, that she would not have done A even if she had willed to do it. If that’s the case, A is not voluntarily substitutable. Broad takes it as evident that voluntary substitutability is a necessary condition for obligability. That is, he takes it as evident that,