Dermot, yes theory is theory, and yes, it’s endless. Theory doesn’t change from instrument to instrument, what changes is how you apply theory to that instrument. For example if I said to play an Am7 chord on acoustic guitar, the way the instrument is built would make the best sounding Am7 chord be in the 1st position using some open strings. However, on an electric guitar you all of a sudden have equally good sounding chords all the way up the neck, and by using theory you can change the inversions and fingerings for that chord. On electric guitar, this is much easier to do technically, but also you have a much more predictable tone and timbre on electric. You might find that playing an Am7 chord on the 5th string in 10th position gets buzzy and frets out on an acoustic, but that won’t happen on an electric as easily. Basically what I’m saying is that depending on how you apply the theory you know, things may get easier or trickier for whichever instrument you’re playing at that moment. But yes, theory is a general set of music logic that is seemingly flawless and can get extremely intricate. I say check out some lessons of mine on theory and begin the long journey for yourself, it’s so rewarding every time you find out why that particular note sounds good there, or why that chord makes sense with all the others.