Halpern-Pearl (HP) version [Halpern and Pearl 2005] here, but our comments apply equally well to the other variants.

It is clear that the structural equations can have a major impact on the conclusions we draw about causality—it is the equations that allow us to conclude that lower air pressure is the cause of the lower barometer reading, and **not the other** way around; increasing the barometer reading will not result in higher air pressure.

The structural equations express the effects of *interventions***: what happens to the** bottle if it is hit with a hammer; what happens to a patient if she is treated with a high dose of the drug, and so on. These effects are, in principle, objective; the structural equations can be viewed as describing objective features of the world. However, as pointed out by Halpern and Pearl [2005] and reiterated by others [Hall 2007; Hitchcock 2001; Hitchcock 2007a], the choice of variables and their values can also have a significant impact on causality. Moreover, these choices **are, to some** extent, subjective. This, in turn, means that judgments of actual causation are subjective.

Our view of actual causation being at least partly subjective stands in contrast to the prevailing view in the philosophy literature, where the assumption **is that the job**
of the philosopher is to analyze the (objective) notion of causation, rather like that of a chemist analyzing the structure of a molecule. This may stem, at **least in part,** from failing to appreciate one of Pearl's lessons: actual causality is only part of the bigger picture of causality. There can be an element of subjectivity **in ascriptions** of actual causality without causation itself being completely subjective. In any case, the experimental evidence certainly suggests that people's **views of causality** are subjective, even when there is no disagreement about the relevant structural equations. For example, a number of experiments show that broadly normative considerations, including the subject's own moral beliefs, affect causal judgment.

(See, for example, [Alicke 1992; Cushman 2009; Cushman, Knobe, and SinnottArmstrong 2008; Hitchcock and Knobe 2009; Knobe and Fraser 2008].) Even in relatively non-controversial cases, people may want to focus on different aspects of a problem, and thus give different answers to questions about causality. For example, suppose that we ask for the cause of a serious traffic accident. A traffic engineer might say that the bad road design was the cause; an educator might focus on poor driver's education; a sociologist might point to the pub **near the** highway where the driver got drunk; a psychologist might say that the cause is the driver's recent breakup with his girlfriend.1 **Each of these answers is reasonable.**
By appropriately choosing the variables, the structural equations framework can accommodate them all.

Note that we said above "by appropriately choosing the variables". **An obvious**
question is "What counts as an appropriate choice?". More generally, what makes a model an appropriate model? While we do want to allow for subjectivity, we need 1This is a variant of an example originally due to Hanson [1958].