Pith and substance[1] is a legal doctrine in Canadian constitutional interpretation used to determine under which head of power a given piece of legislation falls. The doctrine is primarily used when a law is challenged on the basis that one level of government (be it provincial or federal) has encroached upon the exclusive jurisdiction of another level of government.
The Constitution Act, 1867, which established a federal constitution for Canada, enumerated in Sections 91 and 92 the topics on which the Dominion and the Provinces could respectively legislate. Notwithstanding that the lists were framed so as to be fairly full and comprehensive, soon it was found that the topics enumerated in the two sections overlapped, and the Privy Council repeatedly had to rule on the constitutionality of laws made by the federal and provincial legislatures. It was in this situation that the Privy Council evolved the doctrine that, for deciding whether an impugned legislation was intra vires, regard must be had to its pith and substance.
Thus, if a statute is found in substance to relate to a topic within the competence of the legislature, it should be held to be intra vires even though it might incidentally trench on topics not within its legislative competence. The extent of the encroachment on matters beyond its competence may be an element in determining whether the legislation is colourable: whether in the guise of making a law on a matter within its competence, the legislature is, in truth, making a law on a subject beyond its competence. However, where that is not the position, the fact of encroachment does not affect the vires of the law even as regards the area of encroachment.