Imagine something happens. For instance you make a decision. There are three possibilities for this occurence:
- It could be related purely to other factors (determinism)
- It could be not related to other factors (randomness)
- It could be a combination of these (a mixture of determinism and randomness)
But you feel like you have free will (whatever that is - just don't think about it), don't you? Or to put it another way, you feel like your actions are neither determined nor random. You choose them.
And that is precisely why they are determined. They are determined by you. And you already exist to the finest detail at the time you are making the decision. If you made choices (or some element of them) not controlled by your personality, experience, thoughts and anything else that comes under the heading of ‘the state of your brain as a result of genetics and your prior environments’, they would be random, which still isn’t free will (not to mention being a less personal and less appealing model, if that's how you choose your beliefs).
You might argue that you can choose what to think and how to feel , and how heavily to let those things influence you, when making a decision. That doesn't alter the situation however. Those are then choices too, and your decisions for them would presumably have to be made based on other thoughts and feelings , which you would presumably choose, and so on. The point at which free will should have occurred would just be shifted back indefinitely. Again you just have a long chain of cause and effect.
The closest thing you can have to free will is for your actions to be determined purely by the state of your brain. Free will is determinism.