The majority of Rubens's drawings served as a step toward a final work of art in another medium. Rubens kept his drawings close by as studio material to be used by his assistants and collaborators. It was often with the help of his drawings that assistants would execute the related paintings; later, Rubens would merely add the finishing touches. There are indications that the artist guarded his drawings from the outside world, both because he wanted no one to witness his artistic exertions, his sweat and toil, and because the drawings were considered a kind of studio secret. How careful he was about them is clear from his last will and testament, in which he stipulated that his drawings were not to be sold until it was clear that none of his children would become an artist. Rubens himself would never have thought to present them.