 In the projected Fifth Day, Galileo would have treated the power of moving matter to act by impact, which he calls the force of percussion. Ultimately, Galileo was unable to give mathematical principles of this kind of interaction, but this problem subsequently became an important locus of interest. René Descartes, probably following Isaac Beeckman, would eventually convert the problem into the task of finding equilibrium between the forces conserved by colliding bodies. Descartes’s own mathematical treatment was mistaken, but correct principles were given in 1668–9 by Christiaan Huygens, John Wallis, and Christopher Wren.