The monk problem
A monk starts at dawn at the bottom of a mountain and goes up a path to the top, arriving there at dusk. The next morning at dawn he begins to go down the path, arriving at dusk at the place he started from on the previous day. Prove that there is a time of day at which he is at the same place on the path on both days.
Proof: Envision both events occurring on the same day, with a monk starting at the top and another starting at the bottom at the same time and doing the same thing the monk did on different days. They are on the same path, so they must meet each other. The time at which they meet is the time required.
The pons asinorum
Theorem: If a triangle has two equal angles, then it has two equal sides.
Proof: In the figure below, assume angle ABC = angle ACB. Then triangle ABC is congruent to triangle ACB since the sides BC and CB are equal and the adjoining angles are equal.
I considered the monk problem at length in my post Proofs Without Dry Bones. Proofs like the one given of the pons asinorum, particularly its involvement with labeling, recently came up on the mathedu mailing list. See also my question on Math Overflow.
These proofs share a characteristic property; I propose to say they are naive, in the sense Halmos used it in his title Naive Set Theory.
The monk problem proof is naive.
For the monk problem, you can give a model of a known mathematical type (for example model the paths as smoothly parametrized curves on a surface) and use known theorems (for example the intermediate value theorem) and facts (for example that clock time is cyclical and invariant under the appropriate mapping) to prove it. But the proof says nothing about that.
You could imagine inventing an original set of axioms for the monk problem, giving axioms for a structure that are satisfied by the monk’s journeys and their timing and that imply the result. In principle, these could be very different from multivariable calculus ideas and still serve the purpose. (But I have not tried to come up with such a thing.)
But the proof as given simply uses directly known facts about clock time and traveling on paths. These are known to most people. I have claimed in several places that this proof is still a mathematical proof.
Every proof is incomplete in the sense that they provide a mathematical model and analyze it using facts the reader is presumed to know. Proofs never go all the way to foundations. A naive proof simply depends more than usual on the reader’s knowledge: the percentage of explication is lower. Perhaps “naive” should also include the connotation that the requisite knowledge is “common knowledge”.
The pons asinorum proof is naive.
This involves some subtle issues. When I first wrote about this proof in the Handbook I envisioned the triangle as existing independently of any embedding in the plane, as if in the Platonic world of ideals. I applied some labels and a relabeling and used a known theorem of Euclid’s geometry. You certainly don’t have to know where the triangle is in order to understand the proof.
That’s a clue. The triangle in the problem does not need to be planar. It is true for triangles in the sphere or on a saddle surface, because the proof does not involve the parallel axiom. But the connection with the absence of the parallel axiom is illusory. When you imagine the triangle in your head the proof works directly for a triangle in any suitable geometry, by imagining the triangle as existing in and of itself, and not embedded in anything.
- How do you give a mathematical definition of a triangle so that it is independent of embedding? This was the origin of my question on Math Overflow, although I muddled the issue by mentioning specific ways of doing it.
- (This is a variant of question 1.) Is there anything like a classifying topos or space for a generic triangle? In other words, a category or space or something that is just big enough to include the generic triangle and from which mappings to suitable spaces or categories produce what we usually mean by triangles.
- Some of the people on mathedu thought a triangle obviously had to have labels and others thought it obviously didn’t. Specifically, is triangle ABC “the same” as triangle ACB? Of course they are congruent. Are they the same? This is an evil question. The proof works on the generic isosceles triangle. That’s enough. Isn’t it? All three corners of the generic isosceles triangle are different points. Aren’t they? (I have had second, third and nth thoughts about this point.)
- You can define a triangle as a list of lengths of edges and connectivity data. But the generic triangle’s sides ought to be (images of) line segments, not abstract data. I don’t really understand how to formulate this correctly.
1. I could avoid discussion of irrelevant side issues in the monk problem by referring to specific times of day for starting and stopping, instead of dawn and dusk. But they really are irrelevant.