What is the meaning of the term “gazing into a crystal ball” or “an estimate based on prophesies”?

In Shapiro v. Dailey, 2012 BCCA 128 at para. 40, the court repeated earlier descriptions of the exercise including, “gazing into a crystal ball”, “an estimate based on prophesies”, and that “there is much more art than science in the process”.

Insofar as possible, the plaintiff should be put in the position that he or she would have been in but for the injuries caused by the defendant’s negligence. The court’s task is to compare the likely future of the plaintiff’s working life, if the accident had not happened, with the plaintiff’s likely future working life following the accident: Gregory v. Insurance Corporation of British Columbia, 2011 BCCA 144 at para. 32.