Federalism and Division of Powers
The architecture
Sections 91 and 92 of the Constitution Act, 1867 divide legislative authority. Section 91 lists federal heads, including the peace, order and good government power and criminal law; section 92 lists provincial heads such as property and civil rights, and matters of a merely local or private nature. Residual power sits with the federal Parliament. Sections 92A, 93, and 95 create shared or qualified provincial powers over resources, education, and agriculture and immigration.
Characterization and pith and substance
Courts identify the true subject matter of a law, its pith and substance, rather than its form. Incidental effects on the other level's jurisdiction are tolerated if they are truly incidental. The double aspect doctrine permits both levels to legislate on the same subject from different perspectives. Ancillary powers allow provisions that would otherwise be ultra vires if they are connected to a valid scheme.
Paramountcy preview
Where valid federal and provincial laws conflict, federal law prevails under section 52. This is distinct from division of powers; a law can be valid yet inoperative.
Traps
Questions often confuse pith and substance with the law's stated purpose, or apply the double aspect doctrine to a conflict rather than a valid concurrent subject. Another trap is treating the POGG power as a general provincial override. Courts have narrowed POGG to national concern, emergency, and gap-filling.
Worked example
A province enacts a health law that incidentally regulates a federal undertaking. If the core is provincial health, the law may be valid; if it trenches on the federal core, interjurisdictional immunity or paramountcy may apply.