The Two Kinds of Causation
To establish negligence, the claimant must show that the defendant caused the loss they are seeking to recover. There are two types of causation which must be proven: factual causation and legal causation.
Establishing Factual Causation
Factual causation is established if ‘but for’ the breach the claimant would not have suffered the loss: Barnett v Chelsea & Kensington Hospital  1 QB 428.
Loss of a Chance
Factual causation must be established on the balance of probabilities. If the claimant cannot establish that it is more likely than not that they would have avoided the loss but for the breach, the claim with normally fail: Wilsher v Essex  1 AC 1074.
It is not normally possible to argue that the claimant should be able to recover for the loss of a chance if they cannot establish ‘but for’ causation: Gregg v Scott  2 WLR 268.
However, it might be possible to recover for the loss of a chance where the breach causes the claimant to lose the chance to negotiate their way out of economic loss: Allied Maples v Simmons & Simmons  4 All ER 907.
Multiple Concurrent Causes
If a claimant is injured by one defendant (‘A’) and is later injured in the same way by another defendant (‘B’), A is only deemed to have caused the injury up until the date of the second injury: Baker v Willoughby  AC 467.
However, it seems that if a defendant injures the claimant and the claimant would have subsequently developed that injury in any event due to natural causes, the defendant remains liable past the date of the natural cause: Jobling v Associated Dairies  AC 794.
Unknown Factual Causes
Causation rules apply differently in industrial disease cases where the claimant cannot show whether their injury was caused by negligent exposure or non-tortious exposure to a harmful substance. Instead, the claimant only needs to show that the employer ‘materially contributed’ to his injury by increasing the risk: McGhee v National Coal Board  1 WLR 1.
Where the claimant has been negligently exposed to a dangerous substance by multiple employers, each is fully liable if they materially contributed to the risk: Fairchild v Glenhaven  3 WLR 89.
Outside of industrial disease cases, these principles do not seem to apply and normal ‘but for’ causation must be established on the balance of probabilities: Wilsher v Essex  1 AC 1074.
Establishing Legal Causation
Legal causation is established if there are no subsequent acts which break the chain of causation. A break in causation is known as novus actus interveniens. What breaks causation depends on whether the subsequent act is an act of nature, a third-party or the claimant.
Act of Nature
A subsequent act of nature will break the chain of causation if it is unforeseeable: Nichols v Marsland (1876) 2 ExD 1.
The act of a third-party will break the chain of causation if it is unforeseeable: Home Office v Dorset Yacht  AC 1004.
Act of the Claimant
A subsequent act of the claimant will break the chain of causation if it is very unreasonable: Sayers v Harlow Urban District Council  1 WLR 623.
‘Unforeseeable’ means improbable or beyond the types of risk which the defendant’s duty was supposed to guard against: Lamb v Camden LBC  2 All ER 408.
A subsequent act or event will not break causation if it is the kind of thing the defendant’s duty was designed to protect against. For example:
If the defendant has a duty to stop the claimant hurting themselves, then suicide or self-harm will not break causation: Reeves v Commissioner of Police of the Metropolis  1 AC 360.
If the defendant had a duty to control a dangerous third party or object, then that third party’s or object’s actions will not break causation: Home Office v Dorset Yacht Co Ltd  AC 1004.