Twenty-six minutes into the United Rugby Championship quarter final on 6 May, Bulls’ flanker Cyle Brink was sin-binned for taking out an opponent who was waiting for a try-scoring pass. Should there have been a penalty try? The laws say: ‘A penalty try is awarded if foul play by the opposition prevents a probable try from being scored, or scored in a better position.’

But referee Jaco Peyper didn’t award a penalty try, apparently because Brink could have prevented the score legally. So the foul play wasn’t what prevented a probable try from being scored.

This is the ‘but for’ causation test. McMahon & Binchy state it concisely: ‘An act is the cause of an event if the event would not have occurred without (“but for”) the act in question. If the event or effect would have occurred without the act in question then the act cannot be deemed to be a cause.’ Law of Torts (4th ed 2013) ¶2.10.

In Kwan & Scheinert’s Oscar-sweeping Everything Everywhere All at Once, there is different version of the universe for every choice people make. The protagonist moves between them, martial arts style. We can think of operating the ‘but for’ test as asking whether there can be a version of the universe which has the (harmful) ‘event’ and doesn’t have the (unlawful) ‘act’. If there is, the act didn’t cause the event. For referee Peyper, Brink prevents the try legally in at least one universe: therefore his illegal tackle didn’t prevent it.

But on this logic, taking the player out early was unnecessary as well as illegal. Most would say that makes the foul worse. But the ref seems to have been — relentlessly — applying ‘but for’ logic, not degree of fault.

However, when it comes to assessing whether one person’s act caused another’s harm, the analysis is laden with policy and moral considerations. There is more to it than ‘but for’.

For example, in Hayes v Minister for Finance the trial court held that an accident was caused by the combined action of a reckless motorcyclist and Gardaí (police) in pursuit. Although it was conceded that Gardaí had a duty of care to other road users, the Supreme Court disagreed with the finding that Gardaí shared some of the blame for the accident, both on the facts and on the policy ground that they must be permitted to use their discretion enforcing road safety.

Causation can be difficult to establish when a worker claims that an illness was caused by workplace conditions. English courts have adopted a menu of tests in these cases: (a) ‘but for,’ (b) material contribution (one of several causes that makes more than a de minimis contribution to the harm, as in Bonnington Castings Ltd v Wardlaw [1956] AC 613), or (c) increasing the risk of harm (McGhee v National Coal Board [1973] 1 WLR 1 (HL), distinguished in Best v Welcome Foundation Ltd [1993] IR 421, 488-89, (SC O’Flaherty J); Fairchild v Glenhaven Funeral Services Ltd.) Heneghan v Manchester Dry Docks Ltd

In Superquinn Ltd v Bray Urban District Council [1998] IEHC 28, Laffoy J said that a crucial question was whether lack of proper maintenance was a ‘material element and a substantial factor’ in causing a dam at Powerscourt to fail, with the result that the River Dargle flooded a supermarket in Bray. That seems a lower bar than ‘but for’.

In Kuwait Airways Corp v Iraq Airways Co the Iraqi government took 10 of the plaintiff’s airplanes during the 1990 invasion of Kuwait and gave them to the defendant, from whom the plaintiff sought damages. The trial court accepted the defendant’s argument that, had it not taken the aircraft when offered them by the Iraqi government, another Iraqi government agency would have, and so its interference with them was not the ‘but for’ cause of the plaintiff’s loss. A majority of the House of Lords held that the plaintiff did not have to satisfy a ‘but for’ test. Lord Nicholls made no bones that in dispensing with the ‘but for’ test ‘the court is primarily making a value judgment on responsibility,’ having regard the purpose that the tort in question seeks to achieve. Lord Hoffman said that, while there must be some causal link, there is no uniform standard of causation: it varies depending on the basis and purpose of liability.

‘Concurrent’ delay to construction projects is another area where this arises. It’s sometimes said that if two events have a (more or less) equal delay impact, neither of them ’caused’ the delay. But if the test is, for example, ‘material element and a substantial factor’ as in Superquinn, that might give a different result. Like in the URC quarter final.