 As we saw in  Section 3.2,  there is a close connection between interventions and counterfactuals; in particular, the antecedents of structural counterfactuals are thought of as being realized by interventions. Nonetheless, Pearl (2009) distinguishes claims about interventions represented by the do operator from counterfactuals. The former are understood in the indicative mood; they concern interventions that are actually performed. Counterfactuals are in the subjunctive mood, and concern hypothetical interventions. This leads to an important epistemological difference between ordinary interventions and counterfactuals: they behave differently in the way that they interact with observations of the values of variables. In the case of interventions, we are concerned with evaluating probabilities such as