I voted for mental, even though "both" is the obvious answer. Obviously there is a large physical part to play - from warmups to so-called "muscle memory" and high skill levels in this category may or may not be necessary, depending on what you play. ... But, if you don't know what you're doing - at least on some level, even if that level is "ok, next I have to play the 7th fret on the thinnest string and hold it for two seconds", you can play as fast or as cleanly as you like, but the only way it will be music is coincidence. The mental aspect is what sets us apart from machines, puts feeling into what we play, and permits us to improvise and compose. Our physical abilities help us translate what we hear in our head, but the head is where it all starts.