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common mistake’ (Lord Westbury in Cooper v Phibbs (1867) LR 2 HL 149, p 170). What is meant by ‘private right’? Is this not a question of law? 3 P is able to prove that, but for D’s negligent failure to diagnose the correct medical condition, he would have had a 25% chance of recovery. In losing this 25% has P lost something that can be classified as damage? Is this a question of fact or law? (Cf McGhee v NCB [1973] 1 WLR 1; Wilsher v Essex AHA [1988] AC 1074.) 4 ‘ZThe distinction [between mistake of fact and mistake of law]…does not turn upon the fact that the person making the payment could not have discovered the true state of affairs about the law any more than about the facts. It turns upon the purely abstract proposition that in principle (and leaving aside the problem of Schrödinger’s cat) the truth or falsity of any proposition of existing fact could have been ascertained at the time, whereas the law, as it was subsequently declared to have been, could not
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Lord Nicholls:… The general principle is that he who asserts must prove. Generally, although there are exceptions, a plaintiff or applicant must establish the existence of all the preconditions and other facts entitling him to the order he seeks…

Where the matters in issue are facts the standard of proof required in noncriminal proceedings is the preponderance of probability, usually referred to as the balance of probability. This is the established general principle…

The balance of probability standard means that a court is satisfied an event occurred if the court considers that, on the evidence, the occurrence of the event was more likely than not. When assessing the probabilities the court will have in mind as a factor, to whatever extent is appropriate in the particular case, that the more serious the allegation the less likely it is that the event occurred and, hence, the stronger should be the evidence before the court concludes that the allegation is established on the balance of probability. Fraud is usually less likely than negligence. Deliberate physical injury is usually less likely than accidental physical injury…

Evidence is the means whereby relevant facts are proved in court. What the evidence is required to establish depends upon the issue the court has to decide. At some interlocutory hearings, for instance, the issue will be whether the plaintiff has a good arguable case. The plaintiff may assert he is at risk of

the defendant trespassing on his land or committing a breach of contract and that, in consequence, he will suffer serious damage. When deciding whether to grant an interlocutory injunction the court will not be concerned to resolve disputes raised by the parties’ conflicting affidavit evidence.