Dreams are great evidence that the brain and "the mind" are material and not some magik-matter. We can see how "thoughts" come "to mind" that we clearly did not decide to have.
In waking life, information is sent to "the mind", and the rest of the brain and filtered through other parts of the brain, but it goes to whatever region(s) constitute "the mind". In response to this information, which is transferred in the form of neural patterns, the bundles of neurons in "the mind" respond to this information just like any machine. And the nurons throughout the brain are arranged in a manner that certain patterns transfer through much faster than others. The pattern for "scary monsterthing dead ahead" transfers very fast because of however the neurons are organized.
It's an incredible machine, and it's all pre-determined as much as an atom is pre-determined. Because the brain is made up of atoms, it behaves as determinstically as an extremely complex arrangement of atoms. So if one wants to make a case that our actions are not pre-determined, it will not be a matter specific to the brain. Consciousness, this "I" is not evidence of anything non-mechanistic. For all we know rocks and electrical currents could have an "I".
But the brain is a part of an organ-system. And the brain includes the gray matter, but also those medulla and pons things, and the nerves which go throughout the body. So the brain is spread throughout the body and most concentrated in the head. This explains the phantom-limb phenomenon.
It's easy to understand determinism, but the brain cannot calculate itself. It can imagine super-tiny particles, and imagine super-tiny particles forming a larger structure. And then it can imagine a series of these discrete structures interacting. It can even imagine, "it's not discrete structures, but really a series of smaller particles interacting", but it cannot calculate all of those particles and all of those interactions. That is, just because you understand how atoms interact, then molecules, then cells, etc., doesn't mean you can direct-process at the cellular level in any meaningful sense.
For example, take a mechanical pencil. If you open up the mechanical pencil, you will see a spring, some things that go in and out, however that stuff works. I can't calculate them so I just assume mechanical pencils have free will. And you'll say "oh there's the eraser, some eraser crud, some of the eraser comes off in the slot, the plastic has some stuff on it. You won't say, "atom 1, atom 2, atom 3, atom 4..." - which would itself be modeling the interactions of particles as "atoms", but I digress.
And so my theory is that consciousness is the brain modeling itself.
That the "you" is just a model of the brain. And "you" react to stimuli, but your reactions are literally "mind-bogglingly" complicated, nobody can determine how you will react to any stimuli, sometimes not even "you" can, and it appears as if "you" are making decisions independent of the world - unlike a car engine which is obviously deterministic. And just because you can't calculate the car engine or know how it works doesn't mean you ascribe it free will.
You've seen diagrams, and you know the combustion of gasoline is deterministic, and even insects have brains that are so simple that their deterministic nature is obvious. Even if you cannot calculate what an ant will do one second to the next, after analyzing it's behavior you will see enough patterns form to know it is deterministic.
But as you get more complicated machines, the calculations become too much and free will is assumed. I know it's not true, but I functionally assume free will when dealing with other humans, even though humans are to varying degrees also predictable. And of course it was determined that I would assume free will from any point in time.
Now I am not certain that determinism is true, but I am certain that humans behave as deterministically as atoms. I know there's lots of stuff coming forward about how time may not be linear and atoms may behave non-deterministically, but they behave deterministically enough to where I see the world as a complex machine. That I assume free will in others makes perfect sense, because this consciousness, this "I", is a result of the brain not being able to calculate itself, and assuming free will of itself.