an evidentiary foundation. The court must be satisfied that
a reasonable person in the plaintiff’s circumstances ought to have
discovered the claim, and the date of such reasonable discovery
must be determined. It is not sufficient for the court to say that
the claim was discoverable “before the expiry of the limitation
period”, without explaining why. It may be that the date of reasonable discoverability can only be determined at a later stage
in the proceedings, at trial or on a summary judgment motion. In
such a case, the motion to add the defendant should be granted,
with leave for the defendant to plead a limitation defence:
Mancinelli, at paras. 31 and 34.
 The evidentiary burden on a plaintiff seeking to add
a defendant to an action after the apparent expiry of a limitation
period is two-fold. First, the plaintiff must overcome the presumption in s. 5(2) that he or she knew of the matters referred to
in s. 5(1)(a) on the day the act or omission on which the claim is
based took place, by leading evidence as to the date the claim
was actually discovered (which evidence can be tested and contradicted by the proposed defendant). The presumption is displaced by
the court’s finding as to when the plaintiff subjectively knew
he had a claim against the defendants: Mancinelli, at para. 18.
To overcome the presumption, the plaintiff needs to prove only
that the actual discovery of the claim was not on the date the
events giving rise to the claim took place. It is therefore wrong to
say that a plaintiff has an onus to show due diligence to rebut the
presumption under s. 5(2): Fennell, at para. 26.
 Second, the plaintiff must offer a “reasonable explanation
on proper evidence” as to why the claim could not have been discovered through the exercise of reasonable diligence. The evidentiary threshold here is low, and the plaintiff’s explanation should
be given a “generous reading”, and considered in the context of
the claim: Mancinelli, at paras. 20 and 24.
(2) The application of the test requires that the claims be
defined and the evidence considered
 Section 5(1)(a) of the Limitations Act sets out the elements
that must be known to a plaintiff before a claim is discovered.
The issue here is not (as in many other cases) the identification of
the respondents’ involvement in the matters at issue. Rather, the
discoverability issue in this case relates to when the appellants
discovered the facts relating to the respondents’ negligence in
Mr. Morrison’s care; more specifically, when, under s. 5(1)(a)( ii)
and ( iii), the appellants knew or ought reasonably to have known
that an act or omission of each respondent caused or contributed
to Mr. Morrison’s injuries.