First, it should be noted that the Emancipation Proclamation was an executive order, so it wasn't an attempt to enter a political debate. So it does not exactly have a thesis as such. It did, however, include a passage that justified Lincoln's power to emancipate slaves in regions that were in a state of rebellion. Basically, Lincoln claimed this power as a "fit and necessary wartime measure," constitutionally justified"by the power in me vested as Commander-in-Chief...in time of actual armed rebellion." So if there is a thesis, it would be that Lincoln had the power to free slaves as commander in chief of the military. It is important to remember, however, that he only freed slaves in regions that were in rebellion, i.e., the Confederate States. Even those regions of the Confederacy that had been occupied by Union forces (the city of New Orleans, for example) were not subject to the Proclamation. So in practical terms, the document's scope was quite narrow. Its enduring significance, aside from elevating the war to encompass the moral issue of slavery, was to open the door for recruitment of African Americans into the United States military.