It is emphatically the province and duty of the judicial department to say what the law is...If two laws conflict with each other, the courts must decide on the operation of each...This is of the very essence of judicial duty.
John MarshallState inspection laws, health laws, and laws for regulating the internal commerce of a State, and those which respect turnpike roads, ferries, &c. are not within the power granted to Congress. ... Inspection laws, quarantine laws, health laws of every description, as well as laws for regulating the internal commerce of a State, and those which respect turnpike roads, ferries, &c., are component parts of this mass. No direct general power over these objects is granted to Congress, and, consequently, they remain subject to State legislation.
John Marshall