Aspen is the remedy for any fear where the cause of the fear cannot be named. At one end of the spectrum the Aspen fear can be no more than a sense of foreboding, an uneasy anxiety that something unpleasant or frightening is going to happen. At the other it can be a real terror, with physical symptoms such as the hair standing on end. At this extreme Rock Rose could also be helpful.
Sometimes people think of Aspen as a night-time fear, as causeless emotions seem to be uncanny. And of course if you were lying awake in a dark room scared of something but not knowing what it was, then Aspen would be the remedy to take.
But in fact Dr Bach said that for fear of the dark itself Mimulus would be the appropriate remedy to take, since the cause of the fear (the dark) can be named. And nameless Aspen fears are just as likely to occur in full sunlight as on a dark night.