In his Cooper Union speech may be found his strongest argument. "If slavery is right, all words, acts, laws, and constitutions against it are themselves wrong, and should be silenced and swept away. If it is right, we cannot justly object to its nationality,--its universality. If it is wrong, they cannot justly insist upon its extension,--its enlargement. All they ask we could readily grant, if we thought slavery right; all we ask they could as readily grant, if they thought it wrong. Their thinking it right, and our thinking it wrong, is the precise fact upon which depends the whole controversy.... Wrong as we think slavery is, we can yet afford to let it alone where it is, because that much is due to the necessity arising from its actual presence in the nation; but can we, while our votes will prevent it, allow it to spread into the national Territories, and to overrun us here in the free States? If our sense of duty forbids this, then let us stand by our duty fearlessly and effectively. Let us be diverted by none of those sophistical contrivances wherewith we are so industriously plied and belabored, contrivances such as groping for some middle ground between the right and the wrong, vain as the search for a man who should be neither a living man nor a dead man; such as a policy of 'don't care,' on a question about which all true men do care; such as Union appeals beseeching true Union men to yield to disunionists; reversing the divine rule, and calling, not the sinners, but the righteous to repentance; such as invocations to Washington, imploring men to unsay what Washington said, and undo what Washington did. Neither let us be slandered from our duty by false accusations against us, nor frightened from it by menaces of destruction to the government nor of dungeons to ourselves. Let us have faith that right makes might, and in that faith let us, to the end, dare to do our duty as we understand it." In a letter dated July 28, 1859, he wrote, "There is another thing our friends are doing which gives me some uneasiness.... Douglas's popular sovereignty, accepted by the public mind as a just principle, nationalizes slavery, and revives the African slave-trade inevitably. Taking slaves into new Territories, and buying slaves in Africa, are identical things, identical rights or identical wrongs, and the argument which establishes one will establish the other. Try a thousand years for a sound reason why Congress shall not hinder the people of Kansas from having slaves, and when you have found it, it will be an equally good one why Congress should not hinder the people of Georgia from importing slaves from Africa."