Legal arrangements rest on behavioural, cognitive, social, and other assumptions regarding their role and function in society and the legal system. The identification and subsequent evaluation of these assumptions is an important task for legal scholarship. In this article, we focus on the identification and categorisation of these assumptions, providing conceptual distinctions and methodological guidance. We distinguish between assumptions about the value(s), norm(s), or interest(s) underlying a legal arrangement, which can be legal or non-legal, and assumptions about the relationship between the legal arrangement and its underlying value(s), norm(s), or interest(s), which can be logical, causal, or contributory. Regarding the identification, we consider explicit references and inference to the best explanation and theory-driven evaluations as possible methods. Inference to the best explanation, we posit, functions as a manner of reconstructing the theory that the person(s) creating a legal arrangement had in mind regarding the place and function of that legal arrangement in society. Given this, we offer a step-by-step approach to reconstructing this theory in use, drawing from theory-driven evaluations and its sources in the social sciences. These distinctions and guidelines can contribute to understanding the context and untangling the complexities involved in identifying the assumptions that underlie legal arrangements.