This essay addresses a foundational topic in applied sciences with interest in design: how do theories inform design? Previous work has attributed theory-use to abduction and deduction. However, design is about creating an intervention, a possible state that does not exist presently, and these accounts fail to explain how theories permit taking this leap. We argue that the practical value of a theory lies in counterfactual thinking. Theories are like “speculation pumps”: they produce (pump) counterfactual thought experiments of the type: If design was <like this>, then interaction would be <like that>. The more valid these thought experiments are and the better they direct the solution of design problems toward desirable and reliable outcomes, the more useful the theory. Counterfactual thinking sheds new light to design methods and, importantly, can reconcile an underlying tension between design sciences and applied sciences.