I think there undoubtedly needs to be a first mover whose causal efficacy is pure act--that is, non-derivative. But I still am having difficulty seeing how one can show the first mover is pure act. Because of this, I'm wondering, could the first mover just be something material then? If the first mover isn't pure act but rather has purely actual causal power, could it just be something material, something in space-time?