In the last episode, Spinny and Leiby showed that for a certain definition of some of the key terms of our common-sense conceptual apparatus, of substance and property, cause and effect, part and whole, it was logically necessary that all substances, that is all really properly existing things, be indivisible and not interact with each other.
This is in a sense quite a direct and intuitive move: if we understand ‘substance’ as ‘what can be understood to exist without implying the existence of anything else’, then the only substance is the total system, not any element of it. Leibniz, who believed in a personal God, could then say that there are a multitude of such insulated systems, one of which is me, but I know that the others exist, and what they are like, because of my faith that God created them and made them exactly the way I think they are (when I’m thinking properly).
Spinoza, sensible enough to reject a God of this sort, instead had to say that only one substance exists (in his words “God or Nature”) of which I and you are simply aspects. I’m going to focus on Spinoza because I don’t believe in God. Now we ask – what lesson can we actually draw from this 17th-century Dutch jew’s work?