This was discussed recently but I cannot find the thread.
The question is concerning Dreams and Nightmares. Are they the stuff of Mentem or Imaginem? Or both?
The reason I ask is that some characters are in the Magic Realm in a Boundary where the stuff of Dreams and Nightmares are made manifest. The debate is whether this should be a Mentem or Imaginem Boundary, or could it be either.
In Mysteryes revised there are Mysteryes to work wth thase part of a dream, and those part is a Imaginem subject, more real and more "msytical". But the Dream capacity herself is a Mentem construct, and you can affect or create dreams in a sleeper or dreamer without any Mystery with mentem spells out of them.
You should see what aspect is more concernent with the story theme and work with that.
The way I see it is that Dreams are Imaginem, but they are naturally produced/affected by the mind, so that mind-affecting magic (Mentem) can indirectly affect dreams by influencing the mind that influences them.