I would like to know the order of complexity of musical intervals (in a melody). I learned about this from an exposure to the Kodaly method, although I have not learned from nor taught using the Kodaly method. But in my reading it introduced the concept of how the intervals are naturally learned. Seems like there should be an ordering. I can see that the first two are a minor third down and a perfect fourth up, but I don't know where to find a list of the intervals from easiest to hardest, treating the up direction and the down direction as separate things.
I realize that this ordering of interval difficultly or learning order may be dependent on the culture of the student. It would be extra cool to understand how this ordering of interval difficulty related to the harmonic series, since the first interval there is a perfect fifth up, which does not match my understanding that a minor third down is the easiest interval.