Old Master drawings today are considered untouchable, almost holy. This was not necessarily the case in the sixteenth and seventeenth centuries, when they were cut to what was considered the proper format and sometimes heavily reworked so that they would be either more salable or would more closely reflect the taste of the time.
Rubens, who was an avid collector, is known to have retouched dozens of Old Master drawings. As the artist was extremely private about his drawings—those he created himself as well as the ones he collected—he probably did so, as the early biographer Roger de Piles wrote, to "stimulate his senses and to heat up his genius." In other words, this retouching was a challenge to his creativity and inventiveness.