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"?