Thus, the certitude of the principles of Descartes's philosophy cannot be used as a proof for defending his rules of motion; and there is room to believe that if Descartes himself had once more examined his Principles without prejudice and by thinking of explanations similar to those I have stated, he would not have believed that the effects of nature had confirmed his rules, and would not have fallen into contradiction by attributing the hardness of bodies only to the state of rest of their parts, and their elasticity with regard to the effort of subtle matter.