The solution to asymmetrical duplication

In the river pollution example of asymmetrical duplication, application of the but-for test would lead to the conclusion that none of the individual units was a cause of the injury in the sense of strong necessity because the remaining six units would still have caused it. The but-for test once again suggests that the injury was ‘uncaused’.

Yet the solution under the NESS test is not as simple as it is in the double-hit hunters scenario. In that scenario, the duplication is symmetrical because each shot is independently sufficient to cause the injury. To avoid the injury, the each shot would have to be absent. But where the duplication is asymmetrical as in this river pollution scenario, each unit is not independently sufficient, so we cannot say of any individual unit that it must be absent if the injury is to be avoided. Since no individual unit is either strongly necessary (a but-for cause) or independently sufficient (symmetrical duplication) its presence or absence seems to be irrelevant to the outcome. Yet the same could be said of each of the seven units, returning us to a position where

85 _{Martin Hogg, ‘Developing Causal Doctrine’ in Richard Goldberg (ed) Perspectives on Causation (Hart Publishing}

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the harm seems to be mysteriously ‘uncaused’, and logically we know this is not true. However, Wright explains that this is not because of a weakness in the NESS test, it is because the NESS test is being misapplied. If the NESS test is applied correctly, each unit is still a cause of the injury:

[E]ach defendant's one unit was necessary for the sufficiency of a set of actual antecedent conditions that included only four of the other units, and the sufficiency of this particular set of actual antecedent conditions was not affected by the existence of two additional duplicative units.86

This is because of the plurality of causes whereby there can be more than one sufficient set for any outcome, and it is possible for more than one set actually to occur. As noted in an earlier section, the philosophical account of causation involves strong sufficiency (or its equivalent, weak necessity) so it does not require independent sufficiency. Instead of asking whether a particular unit would have to be absent for the harm to be avoided (strong necessity), the correct question is whether the set containing that unit would have to be absent for the harm to be avoided (weak necessity). The situation appears complicated. However, this is only because the two sets are not independently sufficient so there is an overlap between sets.

Wright continues to explain that this conclusion is logically the same whether each of the seven units was contributed by a different individual, or whether one person contributed five units and another contributed two units.87 _{Since the units combine in an indecipherable mass the causal}

status of a unit of pollution is not dependent on whether the remaining units were contributed by one person or by many people. The two units contributed by the second person are still ‘necessary for the sufficiency of a set of actual antecedent conditions that included only three of the first defendant’s five units, a set whose sufficiency was not affected by the existence of two

86 _{Wright, ‘Causation’ (n2) 1793.}

87 _{Richard Wright, ‘Once More into the Bramble Bush: Duty, Causal Contribution, and the Extent of Legal}

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additional duplicative units also provided by the first defendant’.88 _{The fact that the two sources}

occur at the same time, or combine to form an indecipherable whole, makes this disaggregation of the five unit contribution into its component units logically possible: if we are able notionally to disaggregate the defendant’s contribution from the other contribution then logically we can also notionally disaggregate the other contribution.

Wright thus argues that the same causal situation exists whether the harmful agent is one that would normally be thought of as existing in ‘units’ or not, so if two or more fires merge to destroy a building, each is a cause of the destruction even if it would have been insufficient on its own, and even if one of the other fires alone would have been sufficient. The second fire ‘was necessary for the sufficiency of a set of actual antecedent conditions which included another fire…that was “at least large enough to be sufficient for the injury if it merged with a fire the size of the second fire”’.89 _{It will be argued below that in principle Wright is correct to assert that}

disaggregation is appropriate even when the harmful agent is something such as fire that would not normally be thought of as existing in units. However the practical example that he uses to support this assertion is inappropriate because the harm in cases of fire may be divisible so it should not be used to support an argument concerning an indivisible harm.90

Wright has since come to question this ‘at least enough’ formulation, describing it as ‘overly demanding’.91 _{This description is surprising given that his ‘at least enough’ formulation seems to}

make the NESS test very inclusive because in cases of duplication it allows any contribution, which is neither strongly necessary nor independently sufficient, to be labelled as a cause by constructing a set containing that contribution and just enough of the other contributions. He explains:

88 _{Wright, ‘Causation’ (n2) 1793.} 89 _{ibid 1793.}

90 _{For example if two fires combine to damage a house it may be the case that one fire would have damaged one}

room, the second fire another room so the damage would be divisible, but if a person was trapped and killed by the fire their death would be an indivisible damage.

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My initial elaborations of the NESS account were overly demanding. I incorporated the weak-necessity requirement in the definition of singular instances of causation. As I have previously stated, this it too restrictive. The weak-necessity requirement is sufficiently incorporated in a properly formulated causal law, which contains in its antecedent only those abstract conditions the instantiation of which is necessary for the sufficiency of the set of conditions that is sufficient for the immediate instantiation of its consequent. When analysing singular instances of causation, an actual condition c was a cause of an actual condition e if and only if c was a part of (rather than being necessary for) the instantiation of one of the abstract conditions in the completely instantiated antecedent of a causal law, the consequent of which was instantiated by e immediately after the complete instantiation of its antecedent, or (as is more often the case) if c is connected to

e through a sequence of such instantiations of causal laws. This formulation of the

requirement for a NESS condition is more straightforward and simpler to apply than my initial formulations, which requires ‘at least so much’ description of actual conditions in some situations in order to (validly) treat other conditions as NESS conditions.92

This is unconvincing. This seems like a looser approach to causation where the NESS test allows us to understand whether a condition is capable of being a cause but does not allow us to determine whether it actually was a cause on a particular occasion. The NESS ‘test’ thus conceived seems to explain causal generalisations but not to actually test whether something was a cause in a particular instance. This would mean that it has little or no value as a legal test where the law requires a causal link to exist between the particular defendant’s negligence and the particular claimant’s loss.

However, Wright’s previous ‘at least enough’ formulation is appropriate in cases of asymmetrical duplication because asymmetrical duplication only arises where the sources of the harmful agent are factually combined into an indecipherable mass. Since the units are physically combined

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together their effects overlap so we are able to construct overlapping sufficient sets of units containing A’s unit and ‘just enough’ of the other units, or B’s unit and ‘just enough’ of the other units. It is impossible to say that five particular units had an effect and the remaining units had no effect because they were all present together and all operated at the same time – they formed an indecipherable whole so the effects of each unit are similarly indecipherable.

The ‘at least enough’ formulation would be unnecessary and inappropriate in other scenarios that do not involve indivisible damage and sources of the harmful agent combining in an indecipherable mass. If only five units of pollution are added to the water, one by A and four by

B then each unit was necessary for the sufficiency of the set, so A’s unit was a cause of the harm

occurring. This is a simple scenario like Barnett because only one sufficient set has actually occurred so there is no duplication. If A added 5 units and B simultaneously added 5 units then the scenario is equivalent to the double-hit hunters case of symmetrical duplication so both A and B would be labelled as causes on the NESS test because they are independently sufficient. If

A added 5 units and B later added 5 units then A’s units may pre-empt B’s units from being

causes so there is no duplication. Likewise if a total of seven units were added one after the other then the first five units may pre-empt the final two units from becoming causes. There is no danger of the ‘at least enough’ formulation being so expansive that it included cases of pre- emption because the NESS test correctly requires the unit to be necessary for a sufficient set of conditions that actually occurred. One ingredient in any sufficient set must be the condition that ‘the damage has not already occurred’. It is impossible to construct a sufficient set containing some of A’s units and some of B’s in a pre-emptive situation like this, because as soon as A’s 5 units have caused the damage we lose the requisite ‘the damage has not already occurred’ condition in relation to any of B’s units.

The NESS test is very inclusive here, for example if one litre of pollution was sufficient to cause the injury and A added a teaspoon (five millilitres) of pollution while other parties added pollution up to a level of several litres, then the NESS test still leads us to the conclusion that

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A’s teaspoon of pollution was a cause of the damage. Yet it is important to remember that this

problem of asymmetrical duplication only arises when a) the harm is indivisible and b) the defendant contributes a source of a harmful agent that combines with other sources to form an indecipherable mass so that they operate simultaneously. It is also important to remember that causation is only one element of legal liability. Although liability would be joint and several, the defendant can seek contribution from the other responsible tortfeasors. Additionally, a defendant is not liable in negligence if his causal contribution was de minimis, so this reflects the idea that interpersonal responsibility is narrower than causal responsibility so a defendant will not be liable merely because he was a cause of the harm.

This section has addressed asymmetrical duplication which, along with symmetrical duplication, only arises in cases of indivisible damage. The next section looks at divisible damage to highlight the potential problems that can arise with the NESS test.