two stable states for an undetermined length of time, until random noise in the circuit makes it converge to one of the stable states. In asynchronous circuits, arbiters are used to make the decision. They guarantee that up to one outcome is selected at any given point in time, but may take an indeterminate (albeit typically extremely short) time to choose. The metastability problem is a significant issue in digital circuit design, and metastable states are a possibility wherever asynchronous inputs (digital signals not synchronized to a clock signal) occur. The ultimate reason the problem is manageable is that the probability of a metastable state persisting longer than a given time interval t is an exponentially declining function of t. In electronic devices, the probability of such an "undecided" state lasting longer than a matter of nanoseconds, while always possible, can be made negligibly low. In popular culture Lewis Cass, the Democratic candidate for president in 1848, was contrasted with Buridan's ass by Abraham Lincoln: "Mr. Speaker, we have all heard of the animal standing in doubt between two stacks of hay, and starving to death. The like would never happen to General Cass; place the stacks a thousand miles apart, he would stand stock still midway between them, and eat them both at once, and the green grass along the line would be apt to suffer some too at the same time." (This being a reference to Cass's support for "popular sovereignty" in the run-up to the Civil War.) Buridan's Donkey, a 1932 French comedy film, is named after the paradox. "Buridan's Ass" is the name of the sixth episode of the first season of the FX television series Fargo. In the Doctor Who novel The Eight Doctors, the Fifth and Eighth Doctors are confronted by a Raston Warrior Robot. The Doctors stand exactly the same distance away from the Robot as it approaches them; unable to decide which to attack first (since the Robot attacks by sensing brain patterns, which are identical in the two Doctors), the Robot shuts down. The lyrics of a song by Devo, the title track from their album Freedom of Choice, describe a similar situation: "In ancient Rome, there was a poem / About a dog who found two bones" who then, unable to choose between the two, "went in circles till he dropped dead." In the 10th season of The Big Bang