What is the cause for the content of a dream?

Well from my knowledge, there are two main ideas as to what determines what's in your dreams. One is that it consists of what we see throughout the day and the thoughts in the back of our mind. The other theory is that dreams are full of interpretable signs and thusly come from some spiritual source.

Well lately I have been pondering this: what if it was both? What if what people saw and thought about sub-consciously were full of interpretable signs? Would this mean that you could interpret your thoughts and the events around you?

I see dreams pretty much as Edgar Cayce explained, that they can be multi-layered in meaning.
There are some basic ones as follows:

1) The reflective type of dreams mirroring back to you, that which you live each day, the reality as you face it, the hopes & dreams you have of your life.
2)They also (the reflective type), can be like a bunch of rather non-meaning type of dreams, and I see this as a computer (our brain in this case) that "files & sorts" out.....discarding and keeping what it needs...sometimes these dreams just don't seem to fit any pattern at all
3)the Nightmares - often can be strictly diet induced (indigestion can cause them)
4)the "Message Dream" as I term it.......means simply the spiritual higher self, imparting a bit of wisdom for us to take.
These are highly interpretive, and very symbolic, so we need to study them more carefully. You will however be able to recognize when one is such, as you will remember it in detail usually. Something big will stand out......and nag you until you figure it out.
5)The "Deja vous" type of dream - or seeing into the future which is VERY rare.
This type is simply showing us there is no time, (we skipped ahead) in the dream state to an "alternate or potential reality" that isn't necessarily going to be realized here, but sometimes is.

My favorite is the Message Dreams.....As they induce evolutionary spiritual growth.

Hope this helps.