Clustered Causal Ordering

Causal arguments are ubiquitous in human reasoning about physical phenomena. Formalization and computer modeling of these arguments is a major goal of qualitative physics. Building causal qualitative models of rich domains like classical thermodynamics has proven to be difficult . The most accessible and widely agreed upon source of knowledge is found in textbooks, usually in the form of noncausal mathematical equations. Iwasaki and Simon's causal ordering procedure addresses this problem by attempting to extract causality directly from mathematical equations . This paper examines the causal ordering procedure from the point of view of qualitative process theory. We illustrate how causal ordering is dependent on the choice of exogenous variables which in turn requires consideration of the many ways in which a system can interact with its surroundings . In other words, it suffers from environmental instability . This paper shows how directly influenced quantities can be used to find environmentally stable causal clusters. Because of their stability, causal clusters can be studied in isolation and used as building blocks in a broad range of environments . We demonstrate our ideas in the domain of thermodynamics by uncovering the causality of the Ideal Gas Law and Joule's temperature-internal energy relation .