yekke2: Presumably, my statement of Descartes being Assur was facetious but existentialism being Assur was serious.
I meant you only definitely exist in your own thoughts. Everything else may or may not exist. But since we can’t be sure of their existence, we must assume they don’t exist.
The Rambam believes that there is no Stirah between independent wills and God’s Will because… well, we can’t understand God’s Will and it takes a Godly Will to comprehend the answer to that. But, in short, other existence is not a Stirah to Yichud because God is what causes all other things to exist and there is nothing else that functions the way God does.