This is a fear of unknown things, general anxiety or
the feeling that something bad will happen, but you don't know what.
The type of people who need aspen are those who tend to be more sensitive to outside influences. Part of them is tuned into other dimensions or worlds, but they are unable to tell the source, so they pick up things that others might not.
I often prescribe this remedy for those who have panic attacks of unknown sources, or demonic visions while meditating.
It's also useful for fear of things that go bump in the night, or uneasy feelings when you walk into a room or meeting someone.
For Empaths:
Aspen is an amazing remedy for the empath. Personally, I believe that everyone should be carrying a bottle around with them.
Every empath, at some point, will suddenly feel terrible for no reason that they fathom. Things might be going along nicely, when all of a sudden they feel that sinking feeling in the pit of their stomach. They get panicky and feel sure that something is wrong, but they can't work out exactly what.
It's very hard for the Aspen Empath to explain to others why they feel this way. Most non empaths will not understand and will look at you blankly, wondering what you are talking about.
Feelings like vague fears, the sense that something isn't quite right, that something terrible is going to happen or that something is wrong are very common.
It is a very potent remedy for an empath who will often pick up on things that are happening, but they have no idea of the cause or the source behind it.