“ Now this independence is freedom in the negative sense, and this self-legislation of the pure, and therefore practical, reason is freedom in the positive sense. ”
 Immanuel Kant, Critique of Practical Reason (1788). copy citation

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“In fact the sole principle of morality consists in the independence on all matter of the law (namely, a desired object) , and in the determination of the elective will by the mere universal legislative form of which its maxim must be capable. Now this independence is freedom in the negative sense, and this self-legislation of the pure, and therefore practical, reason is freedom in the positive sense. Thus the moral law expresses nothing else than the autonomy of the pure practical reason; that is, freedom; and this is itself the formal condition of all maxims, and on this condition only can they agree with the supreme practical law.” source