In 1668, Huygens, Wallis, and Wren published rules for collisions between perfectly elastic and inelastic bodies. However, many denied that these rules provided an adequate explanatory account of collisions. In an early manuscript draft of the Institutions de Physique, Du Châtelet places herself in this tradition. She states that Huygens, Wallis, and Wren found, not “basic laws of the communication of motion,” but only “secondary laws” (369r). One reason she cites is that perfectly elastic collisions aren’t actually found in nature. She adds that the “primary laws” of collision “will be no easier to find than the cause that makes bodies gravitate towards one another.” So this passage draws an interesting parallel between the explanatory status of mechanical collisions and gravitation.
For the online edition of the Paris Manuscript of the Institutions, see here.