Questions: Counterfactual Truth Conditions and Modal Metaphysics

Question 1

You want to evaluate: 'If the moon were made of cheese, the tides would be unaffected.' Standard material conditional logic says this is automatically true because the antecedent is false. What does Lewis's possible-worlds account say instead?

• A. It is also automatically true, for the same reason — false antecedents make all conditionals true on Lewis's view as well
• B. It is evaluated by asking whether, in the closest possible worlds where the moon is made of cheese, the tides are unaffected — the truth depends on the structure of those worlds
• C. It is false, because the consequent does not follow logically from the antecedent
• D. It cannot be evaluated because the antecedent describes a physical impossibility

Answer: (B)

Lewis's account rejects the material conditional's vacuous truth for counterfactuals. Instead, you identify the possible worlds where the antecedent is true that are *most similar* to the actual world, and check whether the consequent holds there. This can give a determinate truth value for counterfactuals with false antecedents — it is not automatically true. The closest cheese-moon worlds presumably preserve the law of gravitation, so the tidal claim's truth turns on physics in those worlds, not on the falsity of the antecedent.

Question 2

Lewis argues that preserving laws of nature matters more than preserving a large spread of particular facts when determining which worlds are 'closest.' Why?

• A. Because laws of nature are simpler to describe, making similarity comparisons more tractable
• B. Because a world with different laws diverges radically from actuality — small differences in particular facts are more like the actual world than a world with altered physics
• C. Because particular facts are unobservable at the level of possible worlds
• D. Because material conditionals only concern lawlike generalizations, not individual facts

Answer: (B)

Lewis's intuition is that two worlds can differ in many particular facts and still be 'close,' but if they differ in their laws of nature they are radically different kinds of worlds. This is because laws of nature govern everything in a world — changing them changes counterfactually infinite facts downstream. A world where one minor event went differently shares laws with the actual world and is therefore much more similar to it than a world with identical particular history but different physical laws. This asymmetry is what makes Lewis's similarity metric non-trivial.

Question 3

On Lewis's account, a counterfactual 'If A had been the case, B would have been the case' is true whenever A is actually false.

• T. True
• F. False

Answer: False

This describes the *material conditional*, not Lewis's counterfactual. The material conditional is vacuously true whenever its antecedent is false — which is exactly the problem Lewis's theory is designed to solve. On Lewis's view, the counterfactual is true only when B holds in the *closest* A-worlds — the possible worlds most similar to the actual world in which A is true. A false antecedent alone says nothing about the truth of the counterfactual; you must examine the modal neighborhood of the actual world.

Question 4

The truth of a counterfactual conditional depends on facts about possible worlds other than the actual world.

• T. True
• F. False

Answer: True

This is the central claim of Lewis's semantics. A counterfactual 'If A had been the case, B would have been the case' is true or false in virtue of facts about how nearby possible worlds are structured — specifically, whether B holds in the A-worlds closest to actuality. The actual world alone cannot settle the matter, since A is false in the actual world and we need to evaluate what would have happened had it been true. This is what makes counterfactual truth irreducibly modal.

Question 5

Why can't counterfactual conditionals be analyzed as material conditionals, and what specific problem does this create for causal reasoning?

Model answer: The material conditional is vacuously true whenever its antecedent is false. Since counterfactual antecedents are always false in the actual world (that's what makes them counterfactual), every counterfactual would come out true — 'If I had skipped breakfast, the sun would have risen in the west' would be as true as 'If I had skipped breakfast, I would have been hungry.' This collapses all causal distinctions: causal claims like 'C caused E because had C not occurred, E would not have occurred' are expressed as counterfactuals, and if all counterfactuals with false antecedents are equally true, you cannot distinguish genuine causes from irrelevant antecedents.

The failure of the material conditional for counterfactuals is not a minor technicality — it undermines the entire project of using counterfactuals to analyze causation. Lewis's solution, which ties truth to facts about the closest possible worlds, gives counterfactuals non-trivial truth conditions. Causal claims can then be distinguished from accidental correlations: C genuinely caused E only if, in the closest worlds where C is absent, E is also absent — not just in any world where C is absent.