Article 1 Section 8 Clause 8 of the US Constitution: [The Congress shall have power] “To promote the progress of science and useful arts, by securing for limit

Article 1 Section 8 Clause 8 of the US Constitution: [The Congress shall have power] “To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries.” It's neither guaranteed to be an effective or necessary means of achieving that objective, but that is undeniably copyright's stated purpose.