proportion in which the factors were effective in producing the injury or which factor was decisive, the law does not require a pursuer or plaintiff to prove the impossible, but holds that he is entitled to damages for the injury if he proves on a balance of probabilities that the breach or breaches of duty contributed substantially to causing the injury. If such factors so operate cumulatively, it is, in my judgment, immaterial whether they do so concurrently or successively. Indeed Lord Simon regarded "material reduction of the risk" and "substantial contribution to the injury" as mirror concepts. Any other conclusion would mean that the defenders were under a legal duty which they could, on the present state of medical knowledge, ignore (page 9). Lord Kilbrandon expressed a similar conclusion as follows: When you find it proved (a) that the defenders knew that to take the precaution reduces the risk, chance, possibility or probability of the contracting of a disease, (b) that the precaution has not been taken, and (c) that the disease has supervened, it is difficult to see how those defenders can demand more by way of proof of the probability that the failure caused or contributed to the physical breakdown ... In the present case, the pursuer's body was vulnerable, while he was bicycling home, to the dirt which had been deposited on it during his working hours. It would not have been if he had had a shower. If showers had been provided he would have used them. It is admittedly more probable that disease will be contracted if a shower is not taken. In these circumstances I cannot accept the argument that nevertheless it is not more probable than not that, if the duty to provide a shower had not been neglected, he would not have contracted the disease. The pursuer has after all, only to satisfy the court of a probability, not to demonstrate an irrefragable chain of causation, which in a case of dermatitis, in the present state of medical knowledge, he could probably never do. Lord Salmon stated: In the circumstances of the present case, the possibility of a distinction existing between (a) having materially increased the risk of contracting the disease, and (b) having materially contributed to causing the disease may no doubt be a fruitful source of interesting academic discussions between students of philosophy. Such a distinction is, however, far too unreal to be recognised by the common law. Market share liability doctrine In the US, Sindell v Abbott Laboratories 26 Cal. 3d 588 (1980) was a class action for personal injuries said to have resulted from pre-natal exposure to the antimiscarriage drug diethylstilbestrol (DES) which had been manufactured by one of a 34