I was just reading through the Calebais adventure and got to thinking:
If ghosts can keep their art scores, and their ability to do magic then can they teach the Art? They can teach other things, and advise .. so it seems plausable.
Now, in an attempt to disprove the notion that they could I broke up 'teaching the art' into two stages.
Intiation. This apprently requires 'rituals of intiaition' which may or may not be possible for a ghost. This I have drawn from bits of true lineages ... so if this first stage is impossible, then a ghost magus could likely not take on an apprentice. This two stage division assumes, of course, that a bright person with the gift can not learn from a book (without the need for others to intiate them), and is posited for the sake of making ghostly teaching harder to apply.
Instruction ... this one I have more trouble falsifying. If a ghost is sane enough, has proper motivation (or is controlled), etc. the knowlege coming from a ghost should be as good as the knowledge coming from a book or a teacher. Given that ghostly warders can teach the things they know, could a ghostly Pater or Mater keep on teaching a valued filius? Could a necromantically inclined magus 'reatin' a ghoslty archmage as teacher? Does the Tremer stronghold, with all of it's magi ghosts, hold millenia of collective experience to be learned from? Could a ghost passionate about writing on the Art still be an author contributing to the order even in death?
So I put it to the community here - let's all develop good arguments for and against this.
(also - please note that I am still waiting for my 5th ed. core rules, and if the ghostly warder virtue already covers this ... then I will be only slightly embarassed )