Callan vs. May was a United States Supreme Court case that dealt with the issue of whether or not an individual could be held liable for damages caused by another person's negligence. The plaintiff, Callan, sued the defendant, May, after his horse and wagon were damaged when they collided with a railroad car owned by May. At trial it was determined that the accident had been caused by negligence on behalf of both parties; however, since Callan had no control over the actions of May he argued that he should not be held responsible for any damages incurred as a result of their collision. The court ultimately ruled in favor of Callan and found him to be free from liability due to lack of fault on his part. This decision established precedent which holds individuals harmless from damage resulting from another party’s negligent behavior if they are unable to exercise control over said behavior.

In Callan v. May, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving federal law. The majority opinion held that it did not, but Justice Grier dissented from this decision and argued that the state court should have been allowed to hear the case. He reasoned that since there were no laws or regulations in place at the time of filing which would prevent such an action, then allowing it to proceed would be consistent with principles of justice and equity. Furthermore, he noted that Congress had previously granted similar authority to other courts in different cases and thus should do so here as well. Ultimately, Justice Grier concluded by stating his belief that if Congress wanted exclusive jurisdiction over such matters they could pass legislation granting them such power; however until they did so states should be able to exercise their own judicial powers when appropriate circumstances arise.