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The four factors used in deciding whether a use is fair use are: (1) the purpose and character of your use, (2) the nature of the copyrighted work, (3) the amount and substantiality of the portion taken, and
(4) the effect of the use upon the potential market.
"The Congress shall have the 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"