Counterfactual: A counterfactual assertion is a
conditional whose antecedent is false and whose consequent describes how the
world would have been if the antecedent had obtained. The counterfactual takes the form of a subjunctive
conditional: “If P had obtained, then Q would have obtained”. In understanding and assessing such a
statement we are asked to consider how the world would have been if the
antecedent condition had obtained.
For example, “If the wind had not reached 50 miles per hour, the bridge
would not have collapsed” or “If the Security Council had acted, the war would
have been averted.” We can ask two
types of questions about counterfactual conditionals: What is the meaning of
the statement, and how do we determine whether it is true or false? A counterfactual conditional cannot be
evaluated as a truth-functional conditional, since a truth-functional
conditional with false antecedent is ipso facto true. (That is, “if P then Q” is equivalent to “either not P or
Q”.) So is it necessary to provide
a logical analysis of the truth conditions of counterfactuals if they are to be
useful in rigorous thought.
There
is a close relationship between counterfactual reasoning and causal
reasoning. If we assert that
“P caused Q (in the circumstances Ci)”, it is implied that we would
assert: “If P had not occurred (in circumstances Ci) then Q would
not have occurred.” So a causal
judgment implies a set of counterfactual judgments. Symmetrically, a counterfactual judgment is commonly
supported by reference to one or more causal processes that would have conveyed
the world from the situation of the antecedent to the consequent. When we judge that the Tacoma Narrows
Bridge would not have collapsed had the wind not reached 50 miles per hour, we
rely on a causal theory of the structural dynamics of the bridge and the
effects of the wind in reaching the consequent.
How
do we assign a truth value to a counterfactual statement? The most systematic answer is to appeal
to causal relations and causal laws.
If we believe that we have a true causal analysis of the occurrence of
Q, and if P is a necessary part of the set of sufficient conditions that bring
Q to pass—then we can argue that, had P occurred, Q would have occurred. David Lewis (1973) analyzes the truth
conditions and logic of counterfactuals in terms of possible worlds (possible
world semantics). A counterfactual
is interpreted as a statement about how things occur in other possible worlds
governed by the same laws of nature.
Roughly: “in every possible world that is relevantly similar to the
existing world but in which the wind does not reach 50 miles per hour, the
bridge does not collapse.” What
constitutes “relevant similarity” between worlds is explained in terms of
“being subject to the same laws of nature.” On this approach we understand the counterfactual “If P had
occurred, Q would have occurred” as a statement along these lines: “P &
{laws of nature} entail Q”. This
construction introduces a notion of “physical necessity” to the rendering of
counterfactuals: given P, it is physically necessary that Q.
Lewis, David K. 1973. Counterfactuals.
Cambridge: Harvard University Press.
Encyclopedia of
Social Science Research Methods, edited by Michael Lewis-Beck (University of
Iowa), Alan Bryman (Loughborough University), and Tim Futing Liao. Sage Publications.