I lectured on Spinoza in my course on 17th and 18th century philosophy every year for forty years. But universalism has nothing to do with Spinoza. Universalism has nothing to do with how many substances
there are.
Universalism claims that my experience being mine is due to nothing other than its quality of first-person immediacy and therefore, since all experience has this quality, all experience is mine and I am—for
that reason alone—every haver of experience.
Universalism makes no claim about what consciousness is or how many conscious beings there are. It just says they are all me, whatever their number, because their experience is immediate.
Let me explain this further. See if what I am saying sounds like Spinoza.
How do you know what thing you are? Surely not by checking an objective description—whether you possess a certain name or a certain origin. For before you can consider any such objective facts about yourself,
you much more simply know that you are that conscious being whose experience is immediate, first-person in its style. You are the one that seems to be at the centre.
But now consider that every conscious being has experience that fully shares that character you thought belonged only to the one that was you—the immediate, first-person character of experience that supposedly distinguishes you from all the others.
All consciousness in every conscious thing is equally immediate and first-person.
The claim of universalism is that I am equally all conscious things but it falsely seems in each that it is the only one that is me because the specific content of experience in each one is cut off from the specific content in all the others.
It everywhere falsely seems to me that this is the only experience that is immediate and therein mine.
That is what universalism is. Its truth can be demonstrated through both conceptual and statistical arguments. And those arguments have nothing to do with the arguments employed by Spinoza on behalf of his very different claims.