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 found which the abstract principle of law has attached to its being. When we have said this, we have, according to the prevailing theory, completely exhausted their relation to one another. But this view is altogether one-sided. It lays stress exclusively on the dependence of the concrete law on the abstract, but overlooks the fact that there is, just as much, a similar relation of dependence in the opposite direction. Concrete law not only receives life and strength from abstract law, but gives it back, in turn, the life it has received. It is of the nature of the law to be realized in practice. A principle of law never applied in practice, or which has lost its force, no longer deserves the name; it is a worn-out spring in the machinery of the law, which performs no service and which may be removed without changing its action in the least. This applies without limitation to all parts of the law—to the law of nations as well as to private and criminal law; and the Roman law has given it its express sanction, inasmuch as it considers desuetudo as an