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that. They only say how bodies will move after you
have said what the forces are. They permit almost any
forces, and therefore any motions. Only, the principle
of the conservation of energy forces requires us to
explain certain kinds of motions by special
hypotheses about molecules and the like. Thus, in
order that friction and the viscosity of gases should not disobey that
law we have to suppose that gases have a certain
molecular constitution. Setting dynamical laws to
one side, then, as hardly being positive laws, but rather
mere formal principles, we have only the laws of gravitation,
elasticity, electricity and chemistry. Now who will deliberately
say that our knowledge of these laws is sufficient to
make us reasonably confident that they are absolutely eternal and
immutable, and that they escape the great