Where a State law is challenged as having intruded into a Central legislative field, the court examines whether, in PITH AND SUBSTANCE, the enactment falls within the State's competence. This doctrine is known as:

The doctrine of PITH AND SUBSTANCE is applied to determine the true nature and character of a statute when its validity is questioned on the ground of legislative competence. The court looks at the dominant purpose and real substance of the enactment - its pith and substance - rather than incidental trenching, to decide whether it falls in substance within the impugned legislature's field. This allows the court to preserve legislation the incidental encroachment of which on another field is ancillary. Colourable legislation and severability are related but distinct doctrines. Pith and substance is the foundational technique of federal-competence adjudication.