The way equity operated is well known. When the common law courts reached a particularly harsh result flowing
from the demands of formality and rigorous logic, chancery
courts could intervene and mitigate the result in certain circumstances by exercising authority over the defendant’s conscience
and compelling the defendant not to act on his or her full legal
 Law claims the virtues of certainty and predictability,
while equity claims the virtues of doing justice and upholding
fairness in particular cases. As my colleague observes, the
ancient criticism of equity is that its mercies varied arbitrarily
with the length of the Chancellor’s foot.8 The fundamental
tension lives on, even though law and equity have been fused in
Ontario since the late 19th century.
 The constructive trust cases show this tension. Judges
imposing constructive trusts sometimes cite the magnanimous
words of Lord Denning in Hussey v. Palmer,  3 All E.R.
744,  1 W.L.R. 1286 (C.A.), who spoke of a constructive
trust, at p. 747 All E.R., as
a trust imposed by law whenever justice and good conscience require it. It is a
liberal process, founded on large principles of equity, to be applied in cases
where the defendant cannot conscientiously keep the property for himself
alone, but ought to allow another to have the property or a share in it.
 The tension between the legal and equitable impulses is
evident in Pettkus v. Becker,  2 S.C.R. 834,  S.C.J.
No. 103. Dickson J., at pp. 847-48 S.C.R., speaking for the major-
ity, described the attributes of a remedial constructive trust:
The principle of unjust enrichment lies at the heart of the constructive
trust. “Unjust enrichment” has played a role in Anglo-American legal writ-
ing for centuries. Lord Mansfield, in the case of Moses v. Macferlan [(1760),
2 Burr. 1005] put the matter in these words: “the gist of this kind of action
is, that the defendant, upon the circumstances of the case, is obliged by the
ties of natural justice and equity to refund the money”. It would be undesira-
ble, and indeed impossible, to attempt to define all the circumstances in
which an unjust enrichment might arise. . . . The great advantage of ancient
principles of equity is their flexibility: the judiciary is thus able to shape these
malleable principles so as to accommodate the changing needs and mores of
society, in order to achieve justice. The constructive trust has proven to be a
useful tool in the judicial armoury.
(Citations omitted and emphasis added)
 However, Martland J., in dissent, saw the majority’s
extension of the constructive trust remedy in Pettkus as
8 Mitchell McInnes, The Canadian Law of Unjust Enrichment and Restitution (Markham, Ont.: LexisNexis, 2014), at p. 46.