Independent Causes In some situations the claimant has suffered a loss that is known to have been caused by one factor working independently (A or B = loss). A good example of a case to illustrate this is Wilsher v Essex AHA [1988] AC 1074. In this case the plaintiff had been born prematurely. He suffered a condition that caused him to go blind. There was evidence that his blindness could have been caused by any of five different factors, only one of which was tortious (i.e. an excess of oxygen given incorrectly). The others were all the natural result of his premature birth. The evidence suggested that these factors did not operate together to cause the blindness – one factor was solely responsible for the loss. The court applied the ‘but for’ test in this situation. The plaintiff had to prove that but for the defendant’s breach he would not have suffered the blindness. As the standard of proof in civil actions is the balance of probabilities, the plaintiff had to establish that is was more likely than not (i.e. 51 per cent) that the blindness was caused by the negligence as opposed to any of the other possible factors. The plaintiff was unable to do this on the evidence available and so did not succeed.