> [the United States 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.
...
> Some terms in the clause are used in archaic meanings, potentially confusing modern readers. For example, "useful Arts" does not refer to artistic endeavors, but rather to the work of artisans, people skilled in a manufacturing craft; "Sciences" refers not only to fields of modern scientific inquiry but rather to all knowledge.[4]
Copyright is statutory law, meaning it must obey the constitution but otherwise can do anything. (Being an amendment to the preceding body of the Constitution, the First Amendment overrides the Copyright Clause where and only where the two parts conflict.) The Copyright Clause allows Congress to produce short-term restrictions as the means to fulfill a long-term purpose. Congress can add and remove from copyright, but both actions must be for the purpose of promoting the spread, development, and eventual unrestricted public access to creative works. Emphasis on both "eventual" and "unrestricted". In simpler terms, copyright is a means to ensure that the public eventually gets access to every creative work.
Copyright covers creative expression. Copyright does not apply to facts and ideas, nor does it reward "sweat of the brow" [2]. Patents cover implemented inventions, which are novel implementations of practical ideas. Patents do not apply to creative expression, nor unimplemented ideas. (Well, I think that's how patents are supposed to work. But software patents would beg to differ.)
[1] https://en.wikipedia.org/wiki/Copyright_Clause
[2] https://en.wikipedia.org/wiki/Sweat_of_the_brow#United_State...