Compelling evidence, from multiple levels of schooling, suggests that teachers' knowledge and beliefs about knowledge, knowing, and learning ("i.e.," epistemologies) play a strong role in shaping their approaches to teaching and learning. Given the importance of epistemologies in science teaching, we as researchers must pay careful attention to how we model them in our work. That is, we must work to explicitly and cogently develop theoretical models of epistemology that account for the learning phenomena we observe in classrooms and other settings. Here, we use interpretation of instructor interview data to explore the constraints and affordances of two models of epistemology common in chemistry and science education scholarship: epistemological beliefs and epistemological "resources." Epistemological beliefs are typically assumed to be stable across time and place and to lie somewhere on a continuum from "instructor-centered" (worse) to "student-centered" (better). By contrast, a resources model of epistemology contends that one's view on knowledge and knowing is compiled in-the-moment from small-grain units of cognition called resources. Thus, one's epistemology may change one moment to the next. Further, the resources model explicitly rejects the notion that there is one "best" epistemology, instead positing that different epistemologies are useful in different contexts. Using both epistemological models to infer instructors' epistemologies from dialogue about their approaches to teaching and learning, we demonstrate that how one models epistemology impacts the kind of analyses possible as well as reasonable implications for supporting instructor learning. Adoption of a beliefs model enables claims about which instructors have "better" or "worse" beliefs and suggests the value of interventions aimed at shifting toward "better" beliefs. By contrast, modeling epistemology as "in situ" activation of resources enables us to explain observed instability in instructors' views on knowing and learning, surface and describe potentially productive epistemological resources, and consider instructor learning as refining valuable intuition rather than "fixing" "wrong beliefs".