Many of his logic problems are extensions of classic puzzles. Knights and Knaves involves knights (who always tell the truth) and knaves (who always lie). This is based on a story of two doors and two guards, one who lies and one who tells the truth. One door leads to heaven and one to hell, and the puzzle is to find out which door leads to heaven by asking one of the guards a question. One way to do this is to ask, "Which door would the other guard say leads to hell?". Unfortunately, this fails, as the liar can answer, "He would say the door to paradise leads to hell," and the truth-teller would answer, "He would say the door to paradise leads to hell." You must point at one of the doors as well as simply stating a question. For example, as philosopher Richard Turnbull has explained, you could point at either door and ask, "Will the other guard say this is the door to paradise?" The truth-teller will say "No, " if it is in fact the door to paradise, as will the liar. So you pick that door. The truth-teller will answer "Yes," if it is the door to Hell, as will the liar, so you pick the other door. Note also that we are not told anything about the goals of either guard: for all we know, the liar may want to help us and the truth-teller not help us, or both are indifferent, so there's no reason to think either one will phrase answers such as to provide us with the most optimally available kind of comprehension. This is behind the crucial role of actually pointing at a door directly while asking the question. This idea was famously used in the 1986 film "Labyrinth".