By Azeezah Kanji
When and how is public international law relevant in Canadian courts, if it is ever relevant at all? The nutshell-portable answer is: it’s complicated! – but international law may be relevant in a variety of different contexts, and in a variety of different ways.
Broadly speaking, there are two ways of describing the relationship between domestic law and international law: “monist” and “dualist.” From a monist perspective, international law and domestic law are one unified system: international laws are automatically part of domestic law. Conversely, from a dualist perspective, international law and domestic law are two separate legal systems: while international laws may be valid internationally, they cannot be applied domestically unless they are first given force of law in domestic legislation.
Here’s another, more familiar (or should I say – familial), way of thinking about this. In monism, international law and domestic law are like blood relatives. They are automatically members of the same clan/system – even if international law is sometimes treated like the slightly eccentric distant cousin whose positions often fail to carry the day in family arguments. In dualism, however, domestic and international law are more like potential in-laws. Some legally transformative act must be performed before the two become one: marriage in the one case, domestic implementing legislation in the other.
Monism and dualism are not mutually exclusive. A domestic legal system may treat some sources of international law according to the monist model, and others according to the dualist one. English common law has usually taken a monist approach towards applying customary international law. That is, international customs are generally considered to be part of the law of the land, and may be invoked in courts. In its 2007 decision in R v Hape, the Supreme Court of Canada confirmed that prohibitive rules of customary international law are part of domestic law, so long as they do not conflict with existing Canadian legislation.
Treaties, in contrast, have traditionally been regarded through the lens of dualism. As the Supreme Court of Canada held in the well-known Labour Conventions case, treaties signed by the state do not automatically become law in Canada: Canadian lawmakers must first domestically implement the treaty in question.
The field is somewhat muddied, however, by the Canadian jurisprudence on application of international law. International law scholars Jutta Brunnee and Stephen Toope argue that “there appears to be a trend towards treating all of international law, whether custom or treaty, binding on Canada or not, implemented or unimplemented, in the same manner – as relevant and perhaps persuasive, but not as determinative or, dare we say, obligatory.” In other words, both types of international law – custom and treaty – seem to be “kind of, but not totally” applicable.
While treaties are not domestic law in and of themselves, judges may apply a “presumption of conformity” between domestic and international law. This means that legislation may be read so that it accords with Canada’s international treaty obligations (as far as the text of the law allows, and in the absence of express legislative intent to violate international law). In Baker v Canada, Justice L’Heureux-Dube wrote for the majority: “International treaties and conventions are not part of Canadian law unless they have been implemented by statute . . . Nevertheless, the values reflected in international human rights law may help inform the contextual approach to statutory interpretation and judicial review.”
International law, particularly international human rights law, is also important in interpreting the Charter of Rights and Freedoms. In Slaight Communications Inc v Davidson, the Supreme Court held that the Charter should “generally be presumed to provide protection at least as great as that afforded by similar provisions in international human rights documents which Canada has ratified.” Quite simply, Charter rights should be interpreted to at least meet, if not exceed, analogous international human rights standards. In Slaight, the Court also found that Canada’s international human rights commitments are relevant in interpreting section 1 of the Charter (to determine permissible limitations on Charter rights and freedoms).
On the other hand, the Supreme Court of Canada diminished the force of international law in Suresh v Canada, which concerned deportation to the possibility of torture. The prohibition of torture is the highest level of international norm possible – “jus cogens” (also known as “peremptory”) – meaning that states must never conduct, facilitate, or condone torture. The rule against torture is absolute and universal: no exceptions are permitted. However, in Suresh the Supreme Court did not regard the international law on torture as determinative or obligatory in its own right, but merely used it as a tool for interpreting Canadian law – and found that departure from the international norm might be permissible in exceptional circumstances.
To sum up: while in theory customary international law is applied according to monism, and treaties according to dualism, the situation on the ground is (unsurprisingly) somewhat messier and less consistent.
 Ronald St. J. MacDonald, “International Treaty Law and the Domestic Law of Canada” (1975); Hugh M Kindred and Phillip M Saunders et al, International Law Chiefly as Interpreted and Applied in Canada (Edward Montgomery Publications Limited, 2006) at 187.
 R v Hape  2 SCR 292 at para 39.
 Canada (AG) v. Ontario (AG) [Labour Conventions].
 This may be accomplished in one of several different ways: through enactment of new legislation; through amendment of old legislation to bring it into line with international law; or through retention of pre-existing laws which fulfill the state’s obligations under the treaty.
 Jutta Brunnee and Stephen J Toope, “A Hesitant Embrace: The Application of International Law by Canadian Courts” Canadian Yearbook of International Law (Volume 40) (2002) at 5.
 See, for example: Daniels v White  SCR 517; National Corn Growers Assn v Canada (Import Tribunal)  2 SCR 1324; Ordon Estate v. Grail,  3 SCR 437; Canadian Foundation for Children, Youth and the Law v AG Can  1 SCR 76.
 Baker v Canada  2 SCR 817 at paras 69-70.
 Slaight Communications Inc. v. Davidson  1 SCR 1038. The Supreme Court’s decision in Health Services and Support – Facilities Subsector Bargaining Assn v British Columbia  2 SCR 391 similarly sets international human rights law as the “floor” of Charter protection (at para 70). However, the Court seemed to adopt a slightly different approach in Hape, released only the day before the Health Services judgement: “In interpreting the scope of application of the Charter, the courts should seek to ensure compliance with Canada’s binding obligations under international law where the express words are capable of supporting such a conclusion” (at para 56). As Professor Craig Forcese points out, “under the [ . . . ] rule articulated in Hape, Charter protections would not simply benefit from a minimum content presumption, but would also be subject to any ‘ceiling’ or ‘walls’ implied by any of Canada’s international legal obligations. Similarly, any interpretive ‘floor’ previously indicated by Canada’s international human rights obligations could conceivably be lowered by reference to Canada’s international obligations of a non-human rights character.” Craig Forcese, “Supreme Court of Canada Clouds Rules Governing Role of Customary International Law in Domestic Law and Of International Law in Interpreting Canadian Charter,” 1 February 2009, online:.
 Suresh v Canada (Minister of Citizenship and Immigration)  1 SCR 3 at para 60.