"The Rubens version of the Caravaggio…was made around 1611, only a year or so after Caravaggio died. It is, under U.S. law, clearly a derivative work, and thus would have been illegal under current copyright law unless authorized by Caravaggio's estate: Rubens work would be an unfair copy in this case. Yet as art historians, we see Rubens work as perfectly legitimate In fact, Rubens was doing exactly what artists have done probably since they began making art: copying and adapting what has been created before them...."

"[F]rom time immemorial, artists have adapted works by their contemporaries and by older artists for a variety of reasons. The Romans copied the works of the great Greek sculptors—fortunately for us, as the originals have nearly all disappeared. Michelangelo copied Massaccio and Schongauer, Vasari relates how Del Sarto's copy of Raphael's portrait of Leo X fooled even Giulio Romano, who had a share in the original. Rembrandt sketched Raphael's Castiglione, Van Gogh copied Rembrandt, Millet, and Delacroix, and Picasso frequently copied or made versions of works from the past."
--Peter Walsh, "The Coy Copy: Technology, Copyright, and the Mystique of Images," a paper presented at the Town Meeting on Copyright and Fair Use offered at the annual meeting of the College Art Association, Toronto, Canada, February 1998

The full talk (but without the slides that he used to make his points) is found here.

T.