What does lincoln mean by "by general law, life and limb must be protected; yet often a limb must be amputated to save a life, but a life is never wisely given to save a limb. i felt that measures, otherwise unconstitutional, might become lawful by becoming indispensable to the preservation of the constitution through the preservation of the nation. right or wrong, i assumed this ground, and now avow it. i could not feel that to the best of my ability i had even tried to preserve the constitution, if, to save slavery, or any minor matter, i should permit the wreck of government, country"?