The problem I have with proposing a non-physical substance is that such substance must still have physical properties in order to retain relevance in connection to physical substances. To assert that I am a non-physical mind or soul, interacting with other non-physical minds or souls through the physical world I must maintain that my perceptions are causally altered by physical reality, and that physical reality is causally altered by my volition. If either of these is false, then interaction is impossible, but if the non-physical is subject to causality just as the physical is, what separates it as a distinct substance?