The Interaction problem for Dualism
A substance is fundamental if everything else is made up by it. So if you believe that everything that exists is made up of mind, and matter at the most fundemental layer, then mind is fundemental and matter is also fundemental.
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If you believe that they are both fundamental, then they cannot be made up of the same thing(s) otherwise that thing would be the fundemental thing, that grounds all of reality.