I believe that spiritual things (astral projection, ethereal matter, spirits, angels, etc) interact with the Nature/physical plane, but the main thing seperating Nature and the spirit planes are seperate dimensions.
Conscious mind = Physical body and Physical plane
Subconscious mind = Astral body, dreaming mind, spiritual planes, eternal timelessness
As you probably know, the conscious mind is the mind we use with the brain and with the five earth senses. The subconscious mind controlls what our physical body does automatically (blinking, reflexes, etc) and is our dreaming mind. This is one reason why I believe that people probably have past life memories in their dream state.
It's kind of cool and awesome how spirit interacts with the physical dimension. The main thing is seperate spirit dimensions, though.
It's true.