Free labour has the inspiration of hope; pure slavery has no hope.  The power of hope upon human exertion and happiness is wonderful.  The slave-master himself has a conception of it, and hence the system of tasks among slaves.  The slave whom you cannot drive with the lash to break seventy-five pounds of hemp in a day, if you will task him to break a hundred, and promise him pay for all he does over, he will break you a hundred and fifty.  You have substituted hope for the rod.

And yet perhaps it does not occur to you that, to the extent of your gain in the case, you have given up the slave system and adopted the free system of labour.

A Fragment on Slavery.  July 1854

If A can prove, however conclusively, that he may of right enslave B, why may not B snatch the same argument and prove equally that he may enslave A?  You say A is white and B is black.  It is colour, then; the lighter having the right to enslave the darker?  Take care.  By this rule you are to be slave to the first man you meet with a fairer skin than your own.

You do not mean colour exactly?  You mean the whites are intellectually the superiors of the blacks, and therefore have the right to enslave them?  Take care again.  By this rule you are to be slave to the first man you meet with an intellect superior to your own.

But, say you, it is a question of interest, and if you make it your interest you have the right to enslave another.  Very well.  And if he can make it his interest he has the right to enslave you.

Lincoln’s Reply to Senator Douglas at Peoria, Illinois.  The Origin of the Wilmot Proviso.  October 16, 1854

...  Our war with Mexico broke out in 1846.  When Congress was about adjourning that session, President Polk asked them to place two millions of dollars under his control, to be used by him in the recess, if found practicable and expedient, in negotiating a treaty of peace with Mexico, and acquiring some part of her territory.  A bill was duly gotten up for the purpose, and was progressing swimmingly in the House of Representatives, when a Democratic member from Pennsylvania by the name of David Wilmot moved as an amendment, “Provided, that in any territory thus acquired there shall never be slavery.” This is the origin of the far-famed Wilmot Proviso. It created a great flutter; but it stuck like wax, was voted into the bill, and the bill passed with it through the House.  The Senate, however, adjourned without final action on it, and so both the appropriation and the proviso were lost for the time.

...  This declared indifference, but, as I must think, real, covert zeal, for the spread of slavery, I cannot but hate.  I hate it because of the monstrous injustice of slavery itself.  I hate it because it deprives our republican example of its just influence in the world, enables the enemies of free institutions with plausibility to taunt us as hypocrites, causes the real friends of freedom to doubt our sincerity, and especially because it forces so many good men amongst ourselves into an open war with the very fundamental principles of civil liberty, criticizing the Declaration of Independence, and insisting that there is no right principle of action but self-interest.