  We look more carefully at the modeling of causality using structural
equations. It is clear that the structural equations can have a major impact on
the conclusions we draw about causality. In particular, the choice of variables
and their values can also have a significant impact on causality. These choices
are, to some extent, subjective. We consider what counts as an appropriate
choice. More generally, we consider what makes a model an appropriate model,
especially if we want to take defaults into account, as was argued is necessary
in recent work.
