Mahon v. Justice was a United States Supreme Court case that addressed the issue of whether a state could take away a property owner's rights to use their land for certain purposes without compensating them. The case involved a dispute between the owner of a coal mine in Pennsylvania and the state of Pennsylvania. The state had passed a law that prohibited the owner from mining coal on his land, and the owner argued that this was a taking of his property without just compensation. The Supreme Court held that the state had indeed taken the owner's property without just compensation, and that the owner was entitled to compensation for the taking. The Court reasoned that the state had interfered with the owner's right to use his property in a way that was beneficial to him, and that this interference was a taking of his property. The Court also held that the owner was entitled to compensation for the taking, as the state had deprived him of the use of his property without just compensation. The Court's decision in Mahon v. Justice established the principle that a state cannot take away a property owner's rights to use their land for certain purposes without compensating them. This principle has been applied in numerous cases since then, and it remains an important part of the law today.

In the case of Mahon v. Justice, the Supreme Court was asked to decide whether a deed that conveyed land in West Virginia with an express reservation of mineral rights was valid under state law. The majority opinion held that it was not, and thus the deed did not convey any title to minerals beneath the surface. However, Justice Field dissented from this decision on two grounds: firstly, he argued that there were no clear rules or precedents governing such cases in West Virginia at the time; secondly, he argued that even if there had been such rules or precedents they would have supported upholding the validity of this particular deed's reservation clause. In conclusion, Justice Field believed that since there were no clear legal principles applicable to this situation which could be used as guidance for making a ruling one way or another - and since upholding rather than invalidating a contract should generally be preferred when possible - then it would have been more appropriate for him to uphold rather than reject this particular deed's reservation clause.