The following exercise associates to inhabitants of island of knights and knaves created by Smullyan, where knights forever tell truth and knaves always lie. You encounter two people, Albert and Brian. Find, if possible, what Albert and Brian are if they speak to you in manner explained below.
Albert says 'At least one of us is the knave' and Brian says nothing.
a) Is Albert knight, knave, or could he be either?
b) Is Brian knight, a knave, or could he be either?