GaryD The traditional system goes back centuries. I think your idea of talking about trees is a good explanation of how it works.
I was grasping, but I have used trees, or houses, or any row of "things" multiple times in the past to sort out what this naming system is actually about. In discussions among adults and teens, this has worked. It also solves a problem created by how interval names are created, when we're told that these names describe a "distance" between intervals. That's a glitch, because when we describe actual distance, we measure using standard unchanging units like inches, centimeters, ounces, and the starting point is always zero. People see on the piano that CE involves two whole tones, or four semitones, and "3" doesn't add up (literally). The word "distance" is misleading. (That's what SP had been trying to get at, and why he invented the word "span" for this).
Ok, so we have "distance" and this peculiar way of naming it. A student trying to understand rather than just memorizing stuff can be confused by what they actually perceive. So I wondered whether in real life we have something similar, and yes, we do:
My parents farmed, and when you're out in the field you make do with what's around you, and you're also repeatedly working with familiar landmarks. You may gauge the "distance" for this many trees without ever measuring anything. I may indeed need to stretch a rope or wire and for whatever reason I'm counting out trees, or rocks, or whatever. There are plenty of times in real life that we "measure out" things without every using a unit of measure.
The old naming system we inherited is sort of like the old farmers and their fields and familiar landmarks. Instead of unevenly spaced trees and "1st tree, 2nd tree, 3rd tree" we have unevenly spaced notes, always in the same sequence of spacing regardless of where you start. It is not really "distance" in how we "measure" distance.
An important step may be simply realizing that the interval names are not measured distances in how we understand measurement, but this other thing. And then linking that to actual measurement, whether semitones. Some people do this instinctively. There are two things. This "1st 2nd 3rd" naming thing; the actual distance of intervals.
GaryD ..... The more modern system and the one I always use could be called the sound interval system. And this system only cares about what we hear.
Yes. This touches on "actual distance" (as in standard measurement) and in this, C D# and C Eb are in actuality the same distance apart (because they are) and "on the piano" have the same sound quality. I have seen a few ways of expressing intervals in alternate ways - the one I saw first was by semitones. All of them touch on the reality of our "1st tree, 2nd tree" system with unevenly spaced trees is quite awkward, esp. as music gets less diatonic.
I'm only going as far as the idea of two views: one being how the interval naming system is set up .... so as to be able to use it, and know what it's not - and the "realness" where C D# and C Eb are the same piano keys emitting the same sound quality played on the piano.
And then ... in this thread ....... how to learn to use that system. (My first three posts)