A federalism rule that to judge a law's validity the court looks at its true nature and character (its "pith and substance"), and if that falls within the legislature's assigned field, the law is valid even if it incidentally encroaches on a subject in another list.
It is one of the most asked federalism doctrines, regularly paired with colourable legislation and the legislative lists.
Pith and substance allows incidental encroachment if the law's true character is within competence; colourable legislation strikes down a disguised attempt to exceed competence.
Judge a law by its true nature; incidental encroachment on another list is harmless if its pith and substance is within competence (Balsara, 1951).