A.V. Ravishankar asked:

Why are philosophers interested in problems related to counterfactual conditionals? How can we
formalize counterfactual reasoning?

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A counterfactual is formalised as a conditional. The main issue raised by counterfactuals are
connected to the problem that a counterfactual conditional is contrary to fact.

Counterfactuals pose a difficulty for referential theories of meaning which hold that to understand a
statement is to grasp conditions under which a statement is true by virtue of a determinate state of
affairs.

Take the common example "If Oswald hadn't killed Kennedy, someone else would have". For the
counterfactual to be true some supporting assumption needs to be made, such as Oswald's being
party to a conspiracy so he was not acting alone. It is also possible that some other state of affairs
might make the counterfactual true, such as Kennedy being such a lousy person that anyone would
want to kill him.

The theory of possible worlds offers an answer to this problem even though the nature of possible
worlds is controversial. One possible world approach, that of David Lewis in his book Counterfactuals,
has it that because truth is determinate and we suppose a counterfactual refers to ways in which the
actual world could be different, then the truth of a counterfactual depends on how things stand in
possible worlds 'closest' or most 'similar' to the actual world. The problem is that this has the
immediate consequence that if in this world Kennedy was killed, then the most similar possible world
in which Oswald didn't kill him is the world where someone else did.

Rachel Browne