What is the burden of proof for a breach of contract?

In Red Deer College v. Michaels, 1975 CanLII 15 (SCC), [1976] 2 S.C.R. 324, at p.332, Laskin C.J. approved of the following from Williston on Contracts, vol. 111, 3rd ed. (1968), at p. 312: It seems to be the generally accepted rule that the burden of proof is upon the defendant to show that the plaintiff either found, or by the exercise of proper industry in the search, could have procured other employment of an approximately similar kind reasonably adapted to his abilities, and that in absence of such proof the plaintiff is entitled to recover the salary fixed by the contract. Though stated in the context of a claim for breach of contract, the reasoning is just as valid in the context of assessment of damages in tort.