The logic in the title to this post is lurking in every action alleging negligence, but it is a formidable retort in a medical negligence action.
In Chester v Afshar  UKHL, the Defendant adopted that retort. He had operated on the Plaintiff for lower back pain. He had failed to warn her of a possible dangerous outcome of the operation. That outcome transpired. She became partially paralysed. This was a 3%-4% chance outcome of the operation.
Generally, to secure a victory in such circumstances a Plaintiff will be heard to say that, if the appropriate warning had been given the Plaintiff would have declined the operation. Ms. Chester declined to say that. Instead, she said she would have declined the operation that day. By this she meant she would have taken time to reflect on the terms of such a warning; she would have consulted friends and relations and then she would have decided.
The House of Lords found in her favour. In the majority, Lord Hope stressed that to deny her the verdict would render the requirement on doctors to advise and warn their patients of such risks, “…useless in the cases where it may be needed most”.
The Court conceded that in reaching this verdict, it was adjusting the usual burden on Plaintiffs in medical negligence actions.
The outcome was fair. After all, should the Defendant not, in those circumstances and on those facts, be required to prove that the Plaintiff would, in due course and despite the warning, have proceeded with the operation? In all such cases that is the implicit assumption of such Defendants. What is valid about that assumption?