In a federal system where legislative power is distributed between a national legislature and regional legislatures, a law is sometimes challenged on the ground that the legislature enacting it lacked the competence to do so, because the subject matter properly falls within the exclusive domain assigned to the other level of government. The doctrine of pith and substance provides the method courts use to resolve such a challenge: rather than examining every incidental effect a law might produce, a court looks to the true nature and character of the law -- its pith and substance -- to determine which assigned subject matter the law is, in its essential character, actually about.

Under this doctrine, a law enacted by a legislature that is, in its pith and substance, genuinely within that legislature's own assigned domain remains valid, even if the law incidentally touches upon, or has some effect on, a subject matter assigned exclusively to the other level of government, provided this incidental encroachment is not the actual, essential character of the law itself. A law does not become invalid merely because it produces some side effect on a matter outside the enacting legislature's own domain; what matters is the true, essential subject that the law, examined as a whole, is actually designed to regulate.

To determine the pith and substance of a challenged law, courts examine several factors: the actual legislative purpose behind the law, as reflected in its text and its practical operation; the true legal and practical effect the law produces; and the specific mischief or problem the law was actually enacted to address, rather than merely the label or heading under which the law happens to have been formally enacted. A legislature cannot validly expand its own actual competence merely by giving a law an official title suggesting it falls within its own assigned domain, when the law's true pith and substance, upon proper examination, actually falls within the domain exclusively assigned to the other level of government.

Where a court determines that a law's pith and substance genuinely falls outside the enacting legislature's own assigned domain, and any connection to that legislature's own domain is merely incidental, superficial, or a pretext, the law is invalid for want of legislative competence, regardless of how the law happens to have been formally labelled or titled by the enacting legislature.