This page refers specifically to the concept of free will as defined as the ability to choose, think, and act without the constraint of necessity. This is mainly for convenience, and is not supposed to imply that all models of free will can be criticised in the same way. No argument is provided here for the above definition being the only or best definition of free will, and there are plenty of others, such as the freedom to act according to one's motivations. This page may get revisions to make the ideas expressed here more clear.
Every movement, every number, every falling leaf, arises from an immutable historical situation. Every action follows a previous action, and is not a standalone event. It seems beyond doubt that from the beginning of time to the present moment, everything that occurred was all that could ever have occurred. And likewise, from the present moment to the end of time, there is only one course of events that will be possible. This can be reasoned in more detail anywhere, including in the individual choices that make.
When any person is born then they are born into specific material conditions and with a specific genetic code. What follows is a chain of related events from birth until death. It is impossible to examine why a choice is made in detail without looking into the past. Even if we imagine that a man does something as mundane as stand up from a chair, we must look back from that moment into the past in order to answer why it is that he wanted to do it. He may have been feeling joint pain from sitting down in his particular position for as long as he did, he may have been feeling hungry based on when he last ate and what he eats and so on, he may have heard a sound that he wanted to explore thanks to the personality he has developed over time, he may have scheduled something earlier that required him to move, and the list goes on forever but the point is the same for anything. Each reason that leads him to stand up is a historical reason. Nobody has the ability to alter the history they experience, or to disregard history when they make a decision (without this also being the result of history). There is nothing else to base a decision on, and this makes people slaves to the past.
Is it impossible that the man made a completely spontaneous decision about getting up from his chair? What if the man stood up without thinking about it consciously and without anything in particular to do? The man would still be getting up because of historical conditions, and the point may just need to be made more subtle. No part of the brain is free from history: from the state of the brain, its layout, chemical levels, memories, senses, etc. The subconscious mind is still very much influenced by all of these things as long as one believes that it is still part of the physical brain. Even if an action is difficult to explain in detail or does not seem helpful to the one performing it then it does not mean that it is without a cause.
It is worth looking at the behaviourist BF Skinner's pigeons and how by controlling their material environment, he was able to control their actions too. He administered either rewards or punishments, with a variety of schedules and situations, to generate different effects. He even got pigeons playing ping pong. Skinner's beliefs about free will are much the same as what I want to say here. He claimed that while humans assume their internal states and feelings have started something, there are always external reasons in the actual history. By discovering causes of behavior we can dispose of the imaginary internal causes, and of free will itself.
If you could go back in time to a moment where you made a decision that you regret, where you said yes or no to something that changed your life... If you lost all the memories created since that moment, and all of the memories from right before it felt fresh in your mind... If you assumed the same body from that moment, with the same chemical levels in your brain and the same food in your stomach... You would definitely make the same bad decision all over again. I do not believe most people would dispute this, as they can argue no influence to push them to any other decision. It can be said that the decision was a necessary result of its history, and this can be said of every other decision too.
None of this means to imply that everything can be predicted well, or that we will ever be able to predict everything well. It is determinism, but that does not mean anything is determinable. Humans take thousands of actions every day which dominate their physical presentation but of which they are barely even aware of. As I type, and pause, and type again, I make hundreds of tiny movements that I do not usually think about. The human brain moves incredibly quickly and is not tracked into the future on a piece of cosmic parchment. Nevertheless, all these tiny movements that I make are still the result of history, of the events which built my personality, of me being here to type, the way I learnt to type, my mood, the way I sat down, and so on.
One can also look at chaos behaviour for elaboration on that point. There are many systems which are so complex that their behaviour seems to be random, and where long term predictions have never been managed. Their behaviour is actually fully determined by initial conditions, they are just extremely sensitive to what these conditions are. Pioneer of Chaos Theory, Edward Lorenz, summarized Chaos as 'when the present determines the future, but the approximate present does not approximately determine the future'. From a human perspective there will always be mystery and unpredictability in the world.
Determinism should not have to diminish emotional experiences and memories, which can still be unique and special and sudden to us. Because life is unpredictable, and because we are still human, we can still live lives that feel fulfilling. Desires will not vanish just because we become more aware of how they come about, and the illusion of free will might forever stay intact in the heat of the moment. Pessimism is not at all required by the ideas expressed here, and I hope it does not come across as too negative.
If one wants some real implications of all of this then they should look towards divine judgement. Determinism certainly does not mean that harmful actions should go altogether unjudged by society just for being the 'product of history', as we will still desire collectively to live in a world that is safe. Determinism may incline one towards a belief in rehabilitation, and in fair starting conditions in life, but it cannot force these beliefs. What is written here, however, makes the idea of a creator god judging our actions extremely absurd.
There is an argument that if god knows everything, as many claim he does, he therefore should know in advance that someone will sin and that this ruins the idea of judgement. There is also the argument that if god is all powerful, as many claim he is, he should be able to change sinful hearts and minds. The counter argument commonly used to these points is that 'God still gave humans free will', even if he is able to predict what we will do with it. Here, we have argued that free will is a fabrication, and replaced it with the concept of necessity. The counter argument about free will no longer works. Additionally, a god is not even required to be omniscient any more in the initial argument, and now just has to understand that actions arise from material conditions.
A god who would create a heaven and a hell, understanding all of this, is a cruel god. The man born into suffering and atheism who grows up to be sinful is supposedly sent to eternal torment, and the man born into a Christian family or a chance experience with a persuasive evangelist is supposedly sent to eternal joy. This is essentially unjust, and so a judgemental god becomes very difficult to not judge. If a god like this exists, created the world, created the underworld, and now watches people as they hurtle down a one-track road towards it, they must be something of a sadist. It is hard to justify worship of them.