To begin with the most banal: the music must have extramusical associations, preferably intended by the composer. These associations schould be metaphysical (if not necessarily theological) in intent; & (apparently... the negative case doesn't seem to make an awful lot of artistic sense, although there's nothing conceptually amiss with idea) benign-to-transcendental. (One should, however, clearly differentiate between a mystical composer & a mystical composition) A sense of mystery is also pretty much essential: a straight setting of the Latin mass will generally not be described as mystical (although elements within the setting might be); though a choral vocalise quoting the Deus Irae could. Technically, a mystical piece would try to use compositional devices that achieve the above effects. Absence of obvious form is an (equally) obvious start; so getting rid of clear tonal centers & adopting a loose, rhapsodic structure would generally be the way to go. Another popular device - extremely slow tempos - solves two problems simultaneously: firstly, they give a cathedral spin to a piece (if only because _slow_ as the sound metaphor for historical religion has been imprinted in our brains by other media); while also making it more difficult to perceive connections within the score. As mentioned above: quoting of archaic melodies may also be used to reinforce associations of mysticism. All of this probably sounds incredibly mechanical; but if the intent is communication (as most of the modern mystical composers (Hovhaness, Tavener, Paert, etc) seem to believe, it makes sense for their works to speak clearly (& they do: which is why these three composers sell so well). In spite of - or perhaps: because - of their soaring ideas, all three have shared something of a form-follows-function approach to composition... an approach which makes as much sense than any contemporary aesthetics.... All the best, Robert Clements <[log in to unmask]> <http://www.ausnet.net.au/~clemensr/welcome.htm>