[52] The defendant has discharged the evidential burden of showing that the plaintiff had a pre-existing degenerative spinal condition at the time he was injured in the incident in February 1998. The defendant has failed, however, to discharge the evidential burden in respect of showing what the future effects of that pre-existing condition were likely to have been, if the plaintiff were not injured in each of the three incidents. This is because there must be some reasonable measure of precision in the evidence, as to the future effects of the pre-existing condition, absent the injuries that were sustained. Each of the doctors who were questioned about the future effects of the plaintiff's pre-existing spinal degeneration, but for the subject incidents, conceded that the exercise could not be undertaken with a reasonable measure of precision. Although Dr Toft thought it was likely that the plaintiff would, without the three incidents, have had to give up tyre fitting within two or three years of the incident of 30 May 2000, he acknowledged that he could not disagree with the proposition that it was not possible to predict with any reasonable measure of precision as to whether the plaintiff's low back would have become symptomatic, absent that incident in February 1998.