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;
terms
public domain = published before 1923
published now = life of author + 70 years
fair use
The purpose and character of the use (including whether it is commercial, non-profit, or educational)
The nature of the copyrighted work
The amount and substantiality of the portion to be used
The effect upon the potential market for the copyrighted work