Lincoln hoped to use a well-known figure of speech to help rouse the people to recognition of the magnitude of the ongoing debates over the legality of slavery. His use of this paraphrased metaphor is perhaps clearer when you look at some more of his speech:
"A house divided against itself cannot stand." I believe the government cannot endure permanently half slave and half free. I do not expect the Union to be dissolved — I do not expect the house to fall — but I do expect it will cease to be divided. It will become all one thing or all the other. Either the opponents of slavery will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or its advocates will push it forward till it shall become alike lawful in all the States, old as well as new — North as well as South.
As you can see, in this metaphor, the "house" refers to the Union — to the United States of America — and that house was divided between the opponents and advocates of slavery. Lincoln felt that the ideals of freedom for all and the institution of slavery could not coexist — morally, socially, or legally — under one nation. Slavery must ultimately be universally accepted or universally denied.
