It hardly needs stating that the definition of a legal word or term depends upon the purpose for which it is to be defined. If in framing a generalization designed to state a rule or make a discrimination applicable in a specific topic or field of the law, the courts use specified terms, it by no means follows that they intend those terms to be understood in the same sense in generalizations dealing with problems in another topic or field. The words, substance or substantive and procedure or procedural, have been used most frequently in three separate situations: (1) in dealing with controversies concerning the applicability in one territorial jurisdiction of the law of another territorial jurisdiction, (2) in determining the applicability in a federal court of the law of the state in which the federal court is sitting in a trial between citizens of different states, and (3) in determining the operative effect of a statute enacted after the happening of the event or creation of the condition which is the subject of the action. In all of them the solution is generally expressed in terms of procedure or substance. In none of them does the meaning given the term therein furnish a compelling answer to our question. Consequently, whether a constitutional or statutory provision recognizes or confers or creates the power of a court to regulate procedure, its interpretation requires an elementary analysis of our system of litigation.
Edmund M. Morgan,
Rules of Evidence -- Substantive or Procedural?,
10 Vanderbilt Law Review
Available at: https://scholarship.law.vanderbilt.edu/vlr/vol10/iss3/1