Oneale v. Thornton was a Supreme Court case in which the court ruled that a state could not interfere with contracts made between citizens of different states. The case concerned two individuals, Oneale and Thornton, who had entered into an agreement for the sale of land located in Georgia but owned by Oneale, who lived in South Carolina at the time. After entering into this contract, Georgia passed legislation prohibiting such sales to non-residents without permission from their respective legislatures. The Supreme Court held that this law violated Article I Section 10 Clause 1 of the Constitution (the Contract Clause), which prohibits states from passing laws impairing contractual obligations already formed between parties within or outside its borders. This ruling established important precedent regarding interstate commerce and solidified Congress’s power to regulate it under Article I Section 8 Clause 3 (the Commerce Clause).

In the case of Oneale v. Thornton, Chief Justice Marshall delivered a dissenting opinion in which he argued that the court should not have granted a new trial to Thornton on the basis of newly discovered evidence. He reasoned that since this was an action for ejectment, and no fraud had been alleged or proven by either party, there was no reason why such evidence should be considered at all. Furthermore, Marshall noted that it would be unfair to allow one party to benefit from information they were unaware of during trial while denying another their right to due process. Ultimately, he concluded that granting a new trial based on such evidence would set a dangerous precedent and could lead to further injustice if applied indiscriminately in other cases as well.