Let us be clear that nothing in the natural world is undetermined.
You may wish to make a special case for the workings of your brain, but we'll clear up everything else in the universe before tackling that.
Nothing is uncaused, or is free to act in denial or avoidance of the effect of all other actions and processes with which it is existentially linked.
Take any example of a seemingly unpredictable event. How about a coin toss? The likelihood of a tossed coin landing either heads or tails side up is a virtual synonym for true unpredictability.
But we need to make an important distinction. We use the coin-toss as a means of deciding a course of action, on the grounds that we cannot guess how it will land, and it is as likely to be one way as the other.
But that does not mean it is literally undecided.
It is decided absolutely - from the second the coin leaves your hand, and even before that.
It was decided before coins were ever invented.
We don't know which way it will land, but that does not mean that it could land either way.
The likelihood of how it will land pre-toss is fifty-fifty, our ability to guess is fifty-fifty, but the toss itself is a process like any other.
There can be only one outcome every time the act is performed, and that outcome is determined by the toss itself (and other environmental factors operating upon the coin), which are themselves determined by a previous set of influences and constraints, which were themselves determined, and so on, and so on, all the way back to the First Ever Thing.
So it is possible in theory if not practice to work out what that outcome will be before the coin was even minted.
It is easy to list the most obvious and superficial factors influencing the outcome: who is doing the tossing, with what force, exact height and strength of the toss, where in the world, etc etc.
The important point is that the causal chain is too subtle and complex to be comprehensible to us, thus the act of tossing the coin is a good practical way of deciding between potential courses - but its outcome is predetermined for all that, as therefore might be the course it is supposedly deciding upon.
The brain, it seems to me, is exactly analogous with the coin.
What we experience as free will and personality is in truth the overwhelming complexity and intricacy of the causal processes by which mental experience is created. So dense and numerous are they that what we feel is the experience of absolute freedom of choice.
The only way you can possibly deny this, it seems to me, is by denying causation itself.