LXXIV. Have we attained a knowledge of the cause of the elastic motion, which is the property that makes a wand or a sword-blade which is forcibly bent, recover of itself, the straight form it had before, or if it was naturally bent or crooked, and was opened and made straight by force, makes it return to its original crooked shape? Descartes, for the explanation of this phenomenon, recurs to his common asylum, the impulse of the subtil matter, which not being able to penetrate the pores of the wand or sword-blade on the hollow side, where the pores are closed by the inflexion, by its great efforts to open and penetrate them, causes the wand or sword-blade to recover its former figure. But who does not perceive, that for this purpose, it will be necessary to suppose the subtil matter should be for ever moving in opposite directions, from west to east, and upwards and downwards, &c. for the wand or sword-blade, to whatever aspect you turn the hollow side, recovers its natural figure equally the same? Besides, Descartes supposes the subtil matter to be infinitely fluid, from whence it follows, that it will be impossible to shut the pores in such a manner, as that they will become impenetrable to it. LXXV. Others say, that the same impulse which is impressed on a wand or bow by him that bends it, is the thing which opens it afterwards. But against this opinion, it may be urged in the first place, that he who bends it, does it gradually and by slow degrees; and the impetus with which it opens, is violent and quick. Secondly, the archer who bends a bow, is not possessed of a force equal to that, with which it reverts to its former figure, for that is so great when the arrow is drawn near the head, that it is capable of giving it an impetus, that will pierce a substantial thick body through and through; and how can any one communicate a force or impulse, which is greater than that he possesses?