Thanks Jonathan, much to consider here. It seems you are pointing to a significant issue in the word "substance." With words like substance, and being, and "thinking thing," even "spirit" it seems we start reaching the boundaries of what our language can explain. I do struggle with the idea of Spirit (wind, breath) having properties of mind, thinking, will, intention, and ability to act. Yet, so it seems. I'm reading right now a book called Great Thinkers on Great Questions. You might enjoy it.