The use and teaching of experimentation in psychology ought to accompany a discussion of what is within and what is beyond the reach of the method. I address this question by outlining the necessary restrictions that are prerequisite for conducting an experiment. The restrictions include establishing a fixed goal, the fulfillment of which represents successful participation in the experiment, a finite set of possible expressions for satisfying the goal, and a fixed assignment between each possible expression and how it should be performed. These restrictions entail rules that determine what counts as good participation (evaluation), and rules that determine whether a behaviour counts as participation at all (inclusion). Participants’ conformity to the rules makes experimentation possible and, more importantly, maintains the experimenter’s attention on features of performance. This selective attention, in turn, neglects participants’ capacities for adopting and violating rules, and possible alterations in their goals and interpretations. While these capacities fall beyond the scope of experimental psychology, their recognition is necessary for encountering what is not already understood at the start of a research project.