Editing Raymond Smullyan (section)

==Logic problems==
Many of Smullyan's 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 (1986 film)|Labyrinth]]''.

In more complex puzzles, he introduces characters who may lie or tell the truth (referred to as "normals"), and furthermore instead of answering "yes" or "no", use words which mean "yes" or "no", but the reader does not know which word means which. The puzzle known as "[[the hardest logic puzzle ever]]" is based on these characters and themes. In his Transylvania puzzles, half of the inhabitants are insane, and believe only false things, whereas the other half are sane and believe only true things. In addition, humans always tell the truth, and [[vampire]]s always lie. For example, an insane vampire will believe a false thing (2 + 2 is not 4) but will then lie about it, and say that it is false. A sane vampire knows 2 + 2 is 4, but will lie and say it is not. And ''[[mutatis mutandis]]'' for humans. Thus everything said by a sane human or an insane vampire is true, while everything said by an insane human or a sane vampire is false.

{{anchor|Forever Undecided}}
His book ''Forever Undecided'' popularizes [[Kurt Gödel|Gödel's]] [[incompleteness theorem]]s by phrasing them in terms of reasoners and their beliefs, rather than formal systems and what can be proved in them. For example, if a native of a knight/knave island says to a sufficiently self-aware reasoner, "You will never believe that I am a knight", the reasoner cannot believe either that the native is a knight or that he is a knave without becoming inconsistent (i.e., holding two contradictory beliefs). The equivalent theorem is that for any formal system S, there exists a mathematical statement that can be interpreted as "This statement is not provable in formal system S". If the system S is consistent, neither the statement nor its opposite will be provable in it. See also [[Doxastic logic]].

Inspector Craig is a frequent character in Smullyan's "puzzle-novellas."  He is generally called into a scene of a crime that has a solution that is mathematical in nature.  Then, through a series of increasingly harder challenges, he (and the reader) begin to understand the principles in question. Finally the novella culminates in Inspector Craig (and the reader) solving the crime, utilizing the mathematical and logical principles learned. Inspector Craig generally does not learn the formal theory in question, and Smullyan usually reserves a few chapters after the Inspector Craig adventure to illuminate the analogy for the reader. Inspector Craig gets his name from [[William Craig (philosopher)|William Craig]].{{citation needed|date=January 2019}}

His book ''[[To Mock a Mockingbird]]'' (1985){{sfn|Smullyan|1985}} is a recreational introduction to the subject of [[combinatory logic]].

Apart from writing about and teaching logic, Smullyan released a recording of his favorite baroque keyboard and classical piano pieces by composers such as [[Johann Sebastian Bach|Bach]], [[Domenico Scarlatti|Scarlatti]], and [[Franz Schubert|Schubert]]. Some recordings are available on the Piano Society website, along with the video "Rambles, Reflections, Music and Readings". He has also written two autobiographical works, one entitled ''Some Interesting Memories: A Paradoxical Life''{{sfn|Smullyan|2002}} and a later book entitled ''Reflections: The Magic, Music and Mathematics of Raymond Smullyan''.{{sfn|Smullyan|2015a}}

In 2001, documentary filmmaker [[Tao Ruspoli]] made a film about Smullyan called "This Film Needs No Title: A Portrait of Raymond Smullyan."<ref>{{cite web |title=This Film Needs No Title: A Portrait of Raymond Smullyan |url=https://www.youtube.com/watch?v=wcygp0R9Jp4 |website=YouTube | date=9 October 2020 |access-date=5 March 2022}}</ref>