It’s not going to happen in my lifetime!
Thus logician, philosopher and puzzle-constructor Raymond Smullyan, who died last week at the age of 97.
I started reading Smullyan when I was a teenager; I don’t remember whether I read This Book Needs No Title (about philosophy) or What Is The Name Of This Book? (puzzles) first, but whichever it was started a lifetime of enjoyment of both. His philosophy was decidedly playful. In one of his books he dispenses with the question “Does a dog have the Buddha-nature?” with Of course a dog has the Buddha-nature! You just have to look at some dogs to know that.
His puzzles were deceptively simple and quickly ended up being disguised versions of some very difficult topics in first order logic, combinatory logic, Boolean logic, and so on. Though he was most famous for his “Island of Knights and Knaves” puzzles, where knights can only tell the truth an knaves can only lie, he produced a great many puzzles on other topics. To Mock A Mockingbird, in which combinators are thinly disguised as singing birds, is a particular favourite; it changed my understanding of the fundamentals of computer programming.
Among my favourites of all his puzzles though were his two books of absolutely delightful retrograde chess puzzles. Most chess puzzles consider the future: from this position, what should white play next to ensure a mate? Retro puzzles are not concerned with the future, but rather the past; what had to happen in the game in order to arrive at this position?
Here’s my favourite Smullyan retro, which was on the cover of my copy of Chess Mysteries of the Arabian Nights:
The white king has been removed from the board; your task is to deduce where it goes. There is only one square where the white king can be such that the position is possible to produce in a legal game of chess. (Note that I said legal, and not sensible!)
Leave your thoughts in the comments and I’ll give the answer later this week.