#1
what logical explanation can there be to explain the origin of harmonic and melodic minor?
Melodic minor is unstable for phrasing?

Chords are from th natural minor, right?
#2
the origin of harmonic minor and melodic minor is actually pretty easy to explain. first look at natural minor. we see that harmonic minor is the same thing with a raised seventh. this gives us a nice tone to move to the root. it also allows for the V7 chord to be used in key, which again has a very strong pull to the root. so basicly harmonic minor gives you a nice way to move to the root.

but then you have that step and a half between the b6 and 7. sounds a bit funky, and people didnt like it too much in western music. so they decided to raise the sixth, giving you melodic minor. still have the V7 and tone leading to the root, and we dont have any funny intervals. then descending we use natural minor so the b6 pulls to the fifth better and we dont need the pull to the root from the seventh.

im in a bit of a rush or id explain it better, but you should get the basic idea.
#4
I belive that the melodic minor was invented for four-part hamony type things, where there might be one part going up and one down, and you have to alter cirtain notes to avoid horrid harmonies such as tritones..
#5
I think I understand... except for the last thing on the b6 pulling to the fifth better.

I find the melodic minor unstable for phrasing... and I don't really know which 6th and 7th to use when using the melodic minor
#6
Quote by yan_kong
I think I understand... except for the last thing on the b6 pulling to the fifth better.

I find the melodic minor unstable for phrasing... and I don't really know which 6th and 7th to use when using the melodic minor

well the b6 is a semi-tone away from the fifth. this means that when descending, if you play the sixth (b6 here) it is only a semi-tone away from the fifth. thats a very strong movement because the two notes are close together. going from the natural sixth to the fifth is whole tone, so it isnt as strong a movement. the b6 just feels like it wants to fall to the fifth, so its a very strong movement.