# Question about Lewis' counterfactual theory of causation

post by jbkjr · 2024-06-07T20:15:27.561Z · LW · GW · No comments

This is a question post.

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17 Cleo Nardo
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In reading the SEP entry on counterfactual theories of causation, I had the following question occur, and I haven't been able to satisfactorily resolve it for myself.

An event e is said to causally depend on an event c if and only if e would occur if c were to occur and e would not occur if c were not to occur.

The article makes a point of articulating that causal dependence entails causation (if e causally depends on c, c is a cause of e) but not vice versa. It then defines a causal chain as a fine sequence of events c, d, e,... where d causally depends on c, e on d, and so on, before defining c to be a cause of e if and only if there exists a causal chain leading from c to e.

What I'm having trouble with is understanding how c can cause e according to the given definition without e causally depending on c. If there's a causal chain from c to d to e, then d causally depends on c, and e causally depends on d, so if c were to not occur, d would not occur, and if d were to not occur, e would not occur. But doesn't this directly entail that if c were to not occur, then e would not occur and therefore that e causally depends on c?

So how can c cause e according to the definition without e causally depending on c??

answer by Cleo Nardo · 2024-06-08T18:35:03.118Z · LW(p) · GW(p)

If there's a causal chain from c to d to e, then d causally depends on c, and e causally depends on d, so if c were to not occur, d would not occur, and if d were to not occur, e would not occur

On Lewis's account of counterfactuals, this isn't true, i.e. causal dependence is non-transitive. Hence, he defines causation as the transitive closure of causal dependence.

Lewis' semantics

Let  be a set of worlds. A proposition is characterised by the subset  of worlds in which the proposition is true.

Moreover, assume each world  induces an ordering  over worlds, where  means that world  is closer to  than . Informally, if the actual world is , then  is a smaller deviation than . We assume , i.e. no world is closer to the actual world than the actual world.

For each , a "neighbourhood" around  is a downwards-closed set of the preorder . That is, a neighbourhood around  is some set  such that  and for all  and , if  then . Intuitively, if a neighbourhood around  contains some world  then it contains all worlds closer to than . Let  denote the neighbourhoods of .

Negation

Let  denote the proposition " is not true". This is defined by the complement subset

Counterfactuals

We can define counterfactuals as follows. Given two propositions  and , let  denote the proposition "were  to happen then  would've happened". If we consider  as subsets, then we define  as the subset . That's a mouthful, but basically,  is true at some world  if

(1) " is possible" is globally false, i.e.

(2) or " is possible and  is necessary" is locally true, i.e. true in some neighbourhood .

Intuitively, to check whether the proposition "were  to occur then  would've occurred" is true at , we must search successively larger neighbourhoods around  until we find a neighbourhood containing an -world, and then check that all -worlds are -worlds in that neighbourhood. If we don't find any -worlds, then we also count that as success.

Causal dependence

Let  denote the proposition " causally depends on ". This is defined as the subset

Nontransitivity of causal dependence

We can see that  is not a transitive relation. Imagine  with the ordering  given by . Then  and  but not .

Informal counterexample

Imagine I'm in a casino, I have million-to-one odds of winning small and billion-to-one odds of winning big.

1. Winning something causally depends on winning big:
1. Were I to win big, then I would've won something. (Trivial.)
2. Were I to not win big, then I would've not won something. (Because winning nothing is more likely than winning small.)
2. Winning small causally depends on winning something:
1. Were I win something, then I would've won small. (Because winning small is more likely than winning big.)
2. Were I to not win something, then I would've not won small. (Trivial.)
3. Winning small doesn't causally depend on winning big:
1. Were I to win big, then I would've won small. (WRONG.)
2. Were I to not win big, then I would've not won small. (Because winning nothing is more likely than winning small.)
comment by jbkjr · 2024-06-10T15:18:41.628Z · LW(p) · GW(p)

Thanks so much for this—it was just the answer I was looking for!

I was able to follow the logic you presented, and in particular, I understand that  and  but not  in the example given.

So, I was correct in my original example of c->d->e that

(1) if c were to not happen, d would not happen

(2) if d were to not happen, e would not happen

BUT it was incorrect to then derive that if c were to not happen, e would not happen? Have I understood you correctly?

I'm still a bit fuzzy on the informal counterexample you presented, possibly because of the introduction of probability. For example, I don't understand how not winning big entails not winning something because winning nothing is more likely than winning small. If you did not win big, you might still have won small (even if it's unlikely); I don't understand how the likelihood comes into account.

I wish I had an intuitive example of a c and e where c is a cause of e without e causally depending on c, but I'm struggling to imagine one.

Replies from: strawberry calm
comment by Cleo Nardo (strawberry calm) · 2024-06-10T15:34:20.536Z · LW(p) · GW(p)

Suppose Alice and Bob throw a rock at a fragile window, Alice's rock hits the window first, smashing it.

Then the following seems reasonable:

1. Alice throwing the rock caused the window to smash. True.
2. Were Alice ot throw the rock, then the window would've smashed. True.
3. Were Alice not to throw the rock, then the window would've not smashed. False.
4. By (3), the window smashing does not causally depend on Alice throwing the rock.
Replies from: jbkjr
comment by jbkjr · 2024-06-10T18:24:55.218Z · LW(p) · GW(p)

If I understand the example and the commentary from SEP correctly, doesn't this example illustrate a problem with Lewis' definition of causation? I agree that commonsense dictates that Alice throwing the rock caused the window to smash, but I think the problem is that you cannot construct a sequence of stepwise dependences from cause to effect:

Lewis’s theory cannot explain the judgement that Suzy’s throw caused the shattering of the bottle. For there is no causal dependence between Suzy’s throw and the shattering, since even if Suzy had not thrown her rock, the bottle would have shattered due to Billy’s throw. Nor is there a chain of stepwise dependences running cause to effect, because there is no event intermediate between Suzy’s throw and the shattering that links them up into a chain of dependences. Take, for instance, Suzy’s rock in mid-trajectory. This event depends on Suzy’s initial throw, but the problem is that the shattering of the bottle does not depend on it, because even without it the bottle would still have shattered because of Billy’s throw.

Is the example of the two hitmen given in the SEP article (where B does not fire if A does) an instance of causation without causal dependence?

Replies from: strawberry calm
comment by Cleo Nardo (strawberry calm) · 2024-06-10T19:49:09.506Z · LW(p) · GW(p)

tbh, Lewis's account of counterfactual is a bit defective, compared with (e.g.) Pearl's

comment by Dacyn · 2024-06-09T15:08:11.871Z · LW(p) · GW(p)

I think you want to define to be true if is true when we restrict to some neighbourhood such that is nonempty. Otherwise your later example doesn't make sense.

Replies from: strawberry calm
comment by Cleo Nardo (strawberry calm) · 2024-06-09T15:29:00.442Z · LW(p) · GW(p)

Edit: Wait, I see what you mean. Fixed definition.

For Lewis,  for all . In other words, the counterfactual proposition "were  to occur then  would've occurred" is necessarily true if  is necessarily false. For example, Lewis thinks "were 1+1=3, then Elizabeth I would've married" is true. This means that  may be empty for all neighbourhoods , yet  is nonetheless true at .

Source: David Lewis (1973), Counterfactuals. Link: https://perso.uclouvain.be/peter.verdee/counterfactuals/lewis.pdf

Otherwise your later example doesn't make sense.

Elaborate?