You have no reason to believe any of your speculations; they don't add up.
LOL! One of those speculations was that maybe a first cause with a mind did it. I specifically said I would call that God. So, you don't believe the speculation that God might have done it either?
I better write this down! Be it hereby known to all that you, Mark Quayle, have told us that you don't believe my speculation that God might have been the source behind the universe. LOL!
Actually, I think what you meant to say was that you want only one speculation, God. But why not some of the other speculations? They seem more likely.
You like the idea of a source of the universe that has a mind. You even give that speculation a name: God. OK, lets name one of the other speculations: Hod. Would you like to meet Hod? Hod is eternally self existent, able to initiate cosmic inflation and quantum mechanics. Hod is infinite in space and has all eternity to work. How does Hod differ from God? Hod doesn't have a mind. He has no will or emotions. Hod doesn't think. He just acts. Hod is a name I will give to the possible set of physical existence that could have always existed and started off the processes that started the Big Bang.
And Hod seems more likely than God. For with God, you basically take the essence of Hod and add a mind, which is unnecessary. With all eternity to act, why couldn't Hod create universes? Why does Hod need a brain?
Here is another possible cause: Nod. Who is Nod, you ask? Simple. Nod is as close to nothing as you can get. You see, in our universe it is impossible to have nothing. There are always quantum effects even in what we call empty space. And, it could be that you can't even really get a state of nothing outside our universe. Maybe you always have something. Lets call that minimal state of reality Nod. Maybe Nod did it. See
The Problem with Nothing: Why The Indefensibility of Ex Nihilo Nihil Goes Wrong for Theists • Richard Carrier.
So who done it? God, Hod or Nod? Drum roll, please--I don't know.
Maybe a better question is, how can a mind operate when locked into material? (By 'substance', I'm assuming you are referring to material-as-we-know-it.)
The overwhelming evidence shows that brains think, and are the place where we store our memories. See
Is There Life after Death? - The Mind Set Free
But there is reason to believe that the 'substance' of first cause —i.e. the economy from which first cause operates— is a more substantial sort of thing than the concentration of matter at the Big Bang.
Which first cause are you talking about? God? Hod? Nod? There are so many possibilities.
Your view that a spirit God could have mind without there being matter is pure speculation, yes?
And just because we see 'spirit' as lacking substance and form doesn't mean our temporal view is valid in the larger economy of the omni. That is to say, even if 'spirit' is unsubstantial, we have no reason to assume it cannot have mind.
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Uh, yes we do have reason to doubt that a spirit could have a mind. See
The God Impossible • Richard Carrier