1. principle is designed to resolve the difficulty that arises where it is inherently impossible for the claimant to prove exactly how his injury was caused. It applies, therefore, where the claimant has proved all that he possibly can, but the causal link could only ever be established by scientific investigation and the current state of the relevant science leaves it uncertain
2. D's wrongdoing has materially increased the risk that the C will suffer injury. not just a class of people
3. D's conduct must have been capable of causing the C's injury.
4. C must prove that his injury was caused by the eventuation of the kind of risk created by the D's wrongdoing. (McGhee, but not Wilsher!)
5. C must prove that his injury was caused, if not by exactly the same agency as was involved in the D's wrongdoing, at least by an agency that operated in substantially the same way. A possible example would be where a workman suffered injury from exposure to dusts coming from two sources, the dusts being particles of different substances each of which, however, could have caused his injury in the same way.
6. principle applies where the other possible source of the C's injury is a similar wrongful act or omission of another person, but it can also apply where, as in McGhee, the other possible source of the injury is a similar, but lawful, act or omission of the same D. I reserve my opinion as to whether the principle applies where the other possible source of injury is a similar but lawful act or omission of someone else or a natural occurrence.