  We present a novel formalization of counterfactual conditionals in a
quantified modal logic. Counterfactual conditionals play a vital role in
ethical and moral reasoning. Prior work has shown that moral reasoning systems
(and more generally, theory-of-mind reasoning systems) should be at least as
expressive as first-order (quantified) modal logic (QML) to be well-behaved.
While existing work on moral reasoning has focused on counterfactual-free QML
moral reasoning, we present a fully specified and implemented formal system
that includes counterfactual conditionals. We validate our model with two
projects. In the first project, we demonstrate that our system can be used to
model a complex moral principle, the doctrine of double effect. In the second
project, we use the system to build a data-set with true and false
counterfactuals as licensed by our theory, which we believe can be useful for
other researchers. This project also shows that our model can be
computationally feasible.
