Welsh v. Mandeville and Jamesson was a United States Supreme Court case that dealt with the issue of whether or not an individual could be held liable for debts contracted by another person, even if they had no knowledge of such contracts being made. The court ruled in favor of Welsh, finding that he was not responsible for any debt incurred by his partner without his knowledge or consent. This decision established the principle that individuals are only liable for their own debts and cannot be held accountable for those entered into by others without their involvement or awareness. Furthermore, this ruling also set forth the precedent that ignorance does not excuse one from liability when it comes to contractual obligations; rather, parties must take reasonable steps to ensure they understand all terms before entering into agreements with other persons.

In Welsh v. Mandeville and Jamesson, Chief Justice Marshall delivered a dissenting opinion in which he argued that the court should not have dismissed the case on technical grounds. He believed that it was necessary to consider whether or not there had been an actual breach of contract between the parties before dismissing the case. In his view, if there had indeed been a breach of contract then it would be up to a jury to decide what damages were due from one party to another as compensation for any losses suffered by either side. Furthermore, he argued that even though some of the facts may have been disputed between both sides, this did not necessarily mean that they could be disregarded altogether when considering whether or not there had actually been a breach of contract in this particular instance.