Okay, we'll admit it: Supreme Court decisions aren't the most fun to read. They're long, and they're often not designed to keep the reader exactly entranced. However, given the significance of this case to the "House Divided" speech and the state of the nation at the time it was given, it's worth taking a look at the text.

Roger B. Taney's decision starts with a recap of the previous trials. The Supreme Court had to decide whether the decision of the lower courts was valid. But first, as Taney says, they had to decide whether or not the plaintiff (Scott) was a citizen legally allowed to bring the case before the court.

The question is simply this: Can a negro whose ancestors were imported into this country, and sold as slaves, become a member of the political community formed and brought into existence by the Constitution of the United States, and as such become entitled to all the rights and privileges and immunities guaranteed to the citizen? One of which rights is the privilege of suing in a court of the United States in the cases specified in the Constitution. (Source)

Spoiler alert: Taney thinks the answer to this question is no.

Instead, when pondering whether or not the Founding Fathers meant to include Blacks as citizens, he says,

We think they are not, and that they are not included, and were not intended to be included, under the word 'citizens' in the Constitution, and can therefore claim none of the rights and privileges which that instrument provides for and secures to citizens of the United States. (Source)

Apparently, since their ancestors were subjugated when the Constitution was written, obviously Jefferson, Adams, Washington, and the rest couldn't even fathom Black people being citizens of the U.S. Taney then goes into a long historical tangent to prove this idea, going back to the European nations that first enslaved Africans in the New World.

Oh, and a state can't make someone a citizen…unless it's just for their state.

He's quick to point out, "It is not the province of the court to decide upon the justice or injustice, the policy or impolicy, of these laws," just to interpret the Constitution. (Source)

Taney spends a lot of the decision going over the history of the formation of the United States as evidence for his assertions. Towards the end of the decision, he busts out the second bombshell. Since slaves were considered property, and the federal government was supposed to protect people's right to their property, any federal prohibition of slavery was unconstitutional because it denied citizens their rights to their own property.

… the right of property in a slave is distinctly and expressly affirmed in the Constitution. The right to traffic in it, like an ordinary article of merchandise and property… Upon these considerations, it is the opinion of the court that the act of Congress which prohibited a citizen from holding and owning property of this kind in the territory of the United States north of the line therein mentioned, is not warranted by the Constitution, and is therefore void. (Source)

Taney's decision was received very, very differently in different parts of the country. Technically, the Supreme Court's interpretation had to be law, but it only invalidated federal legislation. States' laws banning slavery still counted.

Reading the actual text of the Dred Scott decision helps clarify the legal arguments used to defend the institution of slavery. People found all sorts of ways to do it. Taney was apparently not even a supporter of slavery, but he still interpreted the Constitution in a way that protected it. The slavery issue was divisive for a reason. It may seem so obvious to us now what the right thing to do would be, but back then people had lots and lots of different opinions.