How can a logic puzzle be defined?

Myself and two others had a debate earlier over what separates a logic puzzle and a riddle.

Friend A was arguing that a logic puzzle was a puzzle that required any kind of logical and lateral thinking, although he didn't define what logical thinking is.

Friend B was arguing that a logic puzzle is one that can be solved with an explicit set of instructions, thus resulting in a computer programme being able to solve a large number of logic puzzles using a single programme.

How the debate started

One of them gave a puzzle for us to solve:

You are in a room with 3 switches. One of the switches powers a light bulb in another room. The only way to check which switch powers the light is by walking into the room to check. You can only walk into the room once, and can not walk back into the room with the switches. How can you find out which switch powers the lightbulb?

The answer is to switch one on, wait a while, switch it off and then switch a different one on. Walk into the room. If the light is warm, you know it was the switch that you turned on and off. If the light is on, you know it is the switch you turned on and did not turn back off. If the lightbulb is neither warm nor on then it is the other switch.

Friend B argued that this is a logic puzzle based on the fact that you had to use his idea of 'logical thinking' and lateral thinking.

Friend A argued that it is not a logic puzzle because of the fact that you have to think qualitatively about the fact that the light bulb will warm up, and it is in fact a riddle of some description. A ridiculously long code would need to be written in order for a computer to solve this puzzle, because the computer would have to have so much information stored in the code in order to think of the fact that a light bulb warms up - i.e. only humans can do that.

Who is correct? (Person B's argument convinced me, whereas person A's argument wasn't really an argument with any basis.)

Answer

Borrowing language from fuzzy set theory, what you have is a crisp, binary decision imposed on a set of more than 2 values, at least one of which is fuzzy. Let me explain.

Your friends implicitly assume that puzzles can be crisply partitioned into logic and non-logic puzzles. This is not the case. There are at least 3 types: puzzles fully answerable by logical manipulation of the given information, puzzles requiring (substantial) logical manipulation but which also contain some lateral thinking, and puzzles that are fully or predominantly lateral thinking with very little formal logic, if any.

Here are some examples:

1. pure logic: Einstein's puzzle, as GentlePurpleRain mentions;


2. some logic, some lateral thinking: your light bulb puzzle; and


3. pure lateral thinking, no formal logic: How did the Police Know?.

Note that the term logic puzzle has been adopted by a certain class of truth-table puzzles, so there is some justification to your Friend A's claim. On the other hand, semantically, it is just as justifiable to include type 2 in the term logic puzzles as it is to consider smart phones to be phones even though they are also cameras and music players, etc.

Type 2 is obviously fuzzy, but there is a degree of fuzziness in the other types as well. Type 1 requires language comprehension, especially wordy forms like Einstein's Puzzle. Type 3 often requires some logical thinking, even if it isn't mentioned in the answer.

Instead of trying to measure the amount of logic in the question, I suggest looking at how many answers can be considered correct. Naively, logic puzzles have at most one correct answer, while non-logic puzzles have more than one correct answer. For example, the Police puzzle has many answers posted which are all consistent with the given information. Some of the answers are even contradictory. This is a hallmark of type 3 puzzles. However, for type 2 puzzles, it might only seem to have one answer because others haven't yet been suggested. For example, instead of relying on heat-dissipation, one could rely on non-instantaneous fade-out of the bulb's filament, at least in principle.

To sum up, logic puzzles constrain the set of correct answers by the information explicitly presented, while non-logic puzzles (for want of a better term) allow the set of correct answers to be constrained (so to speak) by information sourced elsewhere. Under this definition, your light bulb puzzle is a non-logic puzzle.