With increased attention to constitutive explanation, mechanists realized the need for an account of constitutive relevance, a principal for sorting relevant from irrelevant factors in a mechanism (Craver 2007; Ylikoski 2013). A system (S) exhibiting phenomenon (ψ) is composed of many different entities (x), with various properties, engaging in myriad activities (φ) organized together (see Figure 1 above in Section 2). One central research problem is to say which of these entities, activities, and organizational features contribute to the phenomenon and which do not. In a sense, this is a challenge of defining the boundaries of a mechanism: of saying what is and is not in the mechanism.