Originally posted by SLY
I heard too that Slash has got nothing to do with music theory , and he used some modes and harmonic minor scale probably without even knowing what are these called.

Yeah he may not of known the names or even that he was using a particular mode, but thats not the point. The point is the effect and sound each scale/mode has on a chord. Whether it's called lydian, or "ewwgahbugah" doesn't mean anything. Take the notes that are in the lydian scale compared to the major scale. The #4 has a colorful tone when played over a major chord, much better that the natural 4th which is somewhat dull. So knowing to play a #4 over a major to get that colorful sound is the point. I think even Slash understood that as all musicians should. The same goes with all the scales and modes.

If you more into playing with feeling that cool. But I believe that playing with feeling involves alot of understanding to be able to use it properly. Say your playing over a simple major chord with nothing more than just feeling. Now you hit a "b3", can you say you understand what you did and how to continue with it. It may sound very off and weird, and you may find yourself correcting it back to a natural 3rd. If you understood it, you would know you played the major blues note (I know b5 is the blues notes, but over a major chord it's the flat 3rd. Used alot in country music but it's a nice tone to slide into a nice blues lick.) So with that you naturally know the music wants to go into a blues type fill, and thats probably where you wanted to go with feeling. But now you understand it better, so you be able to pull it off faster and may avoid the jumpyness that can sometimes occur.

Sometimes to play with true feeling it does take some theory to make it happen. Excluding theory because your a proud feeling player can limit your potential. So experiment and try things in theory, you may learn alot more than you think. Ultimately better you ability to play with feeling.