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.
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 (within its powers), regard must be had to its pith and substance.
In many circumstances, however, a law that is found to be invalid under the pith and substance analysis may still be saved by using the doctrine of necessarily incidental or ancillary effects.
The doctrine was first articulated in Cushing v. Dupuy, where the Judicial Committee of the Privy Council held that certain rules of civil court procedure could be prescribed under the federal bankruptcy power.
The pith and substance doctrine as applied in the jurisprudence of the Judicial Committee of the Privy Council, effectively the British imperial court of appeal, has been carried to other Commonwealth federations.