Paramountcy and Interjurisdictional Immunity
Paramountcy
When a valid federal law and a valid provincial law conflict, the federal law prevails under section 52. Conflict arises when compliance with both is impossible (operational conflict) or when the provincial law frustrates a federal purpose. The doctrine applies only where both laws are valid. If the provincial law is invalid, paramountcy does not arise; invalidity and inoperability are different conclusions.
Interjurisdictional immunity
This doctrine protects the core of a federal head of power from provincial laws that would impair it. It is narrower than paramountcy and applied sparingly. The leading case, Canada v PHS Community Services, retreated from broad application, holding that impairment must be serious or significant. Interjurisdictional immunity is typically invoked where a provincial law of general application touches a federally regulated core, such as aeronautics or banking. It can also protect provincial cores from federal intrusion, though courts have been cautious.
Distinguishing the doctrines
Paramountcy asks whether valid laws conflict; interjurisdictional immunity asks whether a provincial law impairs a federal core even without conflict. Both are distinct from division of powers, which asks whether a law is valid at all.
Traps
Questions often apply paramountcy to invalid laws, or invoke interjurisdictional immunity too readily. Another trap is treating the doctrine as a general rule that federal law always prevails; it only applies in genuine conflict.
Rule of thumb
First test validity; then test conflict under paramountcy; then test impairment of a core under interjurisdictional immunity.