On what legal basis did Dred Scott sue for his freedom?

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Dred Scott claimed that living for an extended period in areas where slavery was forbidden made him free. This is known as the doctrine of "once free, always free." Scott had lived in Illinois and the Wisconsin Territory, where slavery was prohibited by the Missouri Compromise of 1820. He argued that by residing in these areas with his owner, he had essentially become a free person and was entitled to sue for his freedom. However, the Supreme Court ultimately rejected this argument and ruled that Scott had no legal basis for his claim to freedom.

On What Legal Basis Did Dred Scott Sue for His