  Kant's Second Law of Mechanics, stated in Proposition 3, is that every change in matter has an external cause. (Immediately after this principle, Kant adds in parentheses a version of the law of inertia that is much closer to Newton's: “every body persists in its state of rest or motion, in the same direction, and with the same speed, if it is not compelled by an external cause to leave this state” (4:543). Since Kant's Second Law of Mechanics is not identical to Newton's law of inertia, it would require argument to show that, and by means of what additional assumptions, the former entails the latter.) The proof of the main principle depends on the Second Analogy of Experience (which asserts that all changes occur in accordance with the law of cause and effect and thus entails that every change in matter has a cause) as well as on the further assumption that matter has no internal grounds of determinations (such as thinking and desiring), but rather only external relations in space. In his remark to this proposition, which clarifies this “law of inertia,” Kant explains that inertia is to be contrasted with life or the ability of a substance to determine itself to act from an internal principle. Thus, a body's inertia “does not mean a positive striving to conserve its state” (4:544), but rather what it does not do, its lifelessness.