Fuldu wrote about on element of a good puzzle 
(Solvability), and that reminded me that I wanted to try 
to define what a puzzle is in general. You'd think that I'd have gotten to it 
sooner, since it was one of the motivating ideas behind starting this blog, but 
I was distracted by fiddling with the 
Blogger 
template.
On Google, the most prominent article on the subject is "
What Is a 
Puzzle?" by Scott Kim: 
My favorite definition of "puzzle" came out of a conversation with 
  puzzle collector and longtime friend Stan Isaacs: 
  
    - A puzzle is fun, 
    
 - and it has a right answer.
 
I think this is a good 
start, but it's too limiting. The requirement that a puzzle be "fun" is not only 
far too subjective, it also limits the field to what could be considered "good 
puzzles". At the 
Puzzle Museum (a site that ably documents physical puzzles), 
there's a 
classification of 
mechanical puzzles by James Dalgety & Edward Hordern that includes a 
broader definition: 
A PUZZLE IS A PROBLEM HAVING ONE OR MORE SPECIFIC OBJECTIVES, 
  CONTRIVED FOR THE PRINCIPLE PURPOSE OF EXERCISING ONES INGENUITY AND/OR 
  PATIENCE.
A MECHANICAL PUZZLE IS A PHYSICAL OBJECT COMPRISING ONE OR 
  MORE PARTS WHICH FALLS WITHIN THE ABOVE DEFINITION.
This is closer 
to a definition, but there are still issues with it. For one thing, it 
deliberately encompasses items that are designed solely to test dexterity. For 
another, I am not convinced that something designed solely to exercise patience 
is necessarily a puzzle. The best definition I've found so far comes from "
Toward a Theory of Interactive 
Fiction" by Nick Montfort. In the section "
Puzzles and Their Solution, 
Montfort cites a 
newsgroup 
post by Greg Cox with two requirements: 
  
    - a puzzle has to have an objective 
    
 - a puzzle can't be obvious
 
Later, Montfort adds that a 
puzzle is "a challenge" with these qualities. Montfort was trying to define a 
puzzle in the context of Interactive Fiction, and so more qualifications will be 
needed, but I think this is an excellent place to start. 
(I'm not going to discuss IF very much in this post. 
If you desperately want information about it now, you might want to visit 
Brass 
Lantern.)1. A puzzle 
has an achievable objective. Most definitions of a puzzle include a 
reference to an answer, a solution, or a goal. I like "objective" it's an 
adaptive word. It's a bit broader than Kim's "right answer;" it allows the 
possibility of multiple answers. It accepts that the point of a puzzle might be 
a method, not necessarily an simple answer. I think that the addition of 
"achievable" is a natural one. It's not necessary in the context of Montfort's 
original discussion, but it is in ours, where hoaxers occasional put impossible 
tasks into a form similar to a puzzle.
2. A puzzle is not 
obvious. I'm just going to quote Montfort on this, rather than try to 
create a shallow, just-barely-not-plagiarized version of his excellent analysis: 
Obviously, there may be disagreement about what is "obvious" and 
  what is not, but this criterion at least suggests an independent way of 
  determining what is a puzzle and what isn't, one that does not refer to the 
  author's intentions and the [solver]'s specific knowledge and aspirations. Any 
  typical [solver] should be able to determine what is or isn't a puzzle simply 
  by studying [it], without needing to interview the author or take a survey of 
  other [solvers]. The other factors essential to the determination of 
  "obviousness" should be not the mindset of the author or of a particular 
  [solver], but the culture or subculture within which the work was published  
  along with the conventions of [the puzzle type].
3. A puzzle 
is a challenge from its creator. In "Toward a Theory of Interactive 
Fiction," Montfort draws a useful distinction between puzzles and interesting 
bonuses. Discussing the final puzzle in the game 
Adventure, he claims that the score 
reported by the game 
(349 out of 350 before 
solving this puzzle): 
clearly presents a challenge to the interactor: to get the last 
  lousy point, independent of successfully traversing and winning 
  Adventure. If the interactor had 350 points beforehand and 
  dropping the magazine gave the interactor 351 pointsand there was thus no way 
  to know beforehand that an extra point could be obtainedthis could be 
  referred to as an Easter egg but would not be a puzzle. A challenge would not 
  have been presented initially.
This also means that a puzzle cannot 
occur naturally. It is always artificial, or is artificially framed. If a 
reporter writes an article with three instances of three words that are homonyms 
of each other, it's not a puzzle. But if the reporter, or another person, 
presents the article with the frame "Can you find nine words that sound alike in 
this article", it becomes part of one.
And that's all Montfort gives us. 
Right now, we have a definition of a puzzle that includes a game of chess: the 
objective is to win, it is not obvious how, and there is a challenge presented. 
To distinguish puzzles further, we turn to Greg Costikyan and Chris Crawford who 
tell us . . .
4. A puzzle is static. Both 
The Art 
of Computer Game Design by Crawford and "
I Have No Words and I Must Design" 
by Costikyan are focused on games, and in consequence, I believe both miss the 
boat slightly when defining games. But a puzzle being "static" is a useful idea. 
Many puzzles are clearly static: paper-and-pencil puzzles are clearly so. But 
there are others, like Rubik's Cube or 
sliding-block 
puzzles, that are clearly 
reactive. 
And we already pulled or initial definition from an 
interactive medium. By "static," I mean that a 
puzzle must be predictable. No essential elements are subject to chance. In 
theory, every variable can be accounted for by the solver.
For example, 
in the game 
Deadly Rooms of Deathyou control 
an exterminator who must kill the creatures within a dungeon, room by room. 
These adversaries move according to preset rules. Those rules are not disclosed 
to the player, and the rules are occasionally very complex, but they are fixed. 
In fact, if you start from the same position in a room and duplicate a series of 
moves, the monsters will always respond in the same way. This game is a series 
of puzzles. In contrast, the "puzzle" game Tetris features randomized pieces 
that move at variable speeds, so that the same series of moves will produce 
extremely different results on different plays.
I think the definition is 
almost complete, but there is still a class of items that needs to be 
eliminated. Under this definition, Dalgety & Hordern can still claim that 
toys that test dexterity are puzzles, and Crawford can claim the same for simple 
games. So I add . . .
5. A puzzle does not test physical 
traits. This includes dexterity and hand-eye coordination, as well as 
strength, speed, stamina, height, weight, arm length and other abilities like 
knitting or singing in key. Any of these things might be tied into a puzzle in 
some way 
(some larger puzzle events require 
these kinds of things), but the puzzle is always separate from it. Many 
video-game puzzles feature this kind of distinction. The player may have figured 
out the method that kills the almost entirely invulnerable monster, but he may 
still be unable to actually defeat it if his hand-eye coordination is 
lacking.
I think this is a good working definition. It seems to filter 
games, toys, problems and jokes out of the category of puzzles in a way that I 
find appropriate. But it does leave one gray area: Trivia. I'm fine with that, 
for now, because I don't have a clear idea of where it ought to go. Add that to 
the list of things to think about for this blog. 
Spoilers 
Ahead