Free will as it is usually understood is the ability of an agent to do an intentional action that is at least to some extent independent of influences affecting the agent. This independence is supposed to make the agent the ultimate controller of his actions, which guarantees that the agent is morally responsible for his actions and deserves rewards or punishments for them.
However, an agent that does an intentional action can never be the ultimate controller of this action, because the action is determined by an intention (and possibly also by some influences affecting the agent). For example, if raising of my left arm is my intentional action, then it must be determined by my intention to raise my left arm. But how does the determination of an action by the agent's intention rule out the agent's ultimate control over the action?
To answer that question let us first look at what an intention is. Here we may help ourselves with some definitions in a dictionary, such as the Merriam-Webster dictionary (www.m-w.com). The first definition of the word "intention" in this dictionary is "a determination to act in a certain way", which indicates that an intention is A STATE OF MIND. Another relevant definition of this word is "what one intends to do or bring about", which indicates that an intention is a goal or purpose, AN IDEA one has in mind.
So A STATE OF MIND or AN IDEA determines an intentional action. Then for an agent to be in control of his intentional action it seems that he must determine his state of mind or the idea he has in mind, and through these he can determine the action. However, determining a state of mind or an idea in mind is an action too! It is an action of creating the state of mind or the idea and if THIS action is not intentional then obviously the agent is not in control of it and consequently not in control of the final action determined by the state of mind/idea. On the other hand, if the determining of the state of mind/idea is an intentional action, then it must be determined by an intention to create that state of mind/idea, and this intention is just another state of mind/idea. So we get an infinite regress of states of mind/ideas determining other states of mind/ideas, in which there is no place for an agent's ultimate control of his actions. Or assuming (as we usually do) that the agent's mind has a finite history, we arrive at the first state of the agent's mind or the first idea in his mind, which is not determined by his preceding states of mind/ideas, and therefore it is not intentional, he doesn't have control over it and consequently he doesn't have control of actions determined by it.
And moreover, it seems doubtful that it is possible to even have an intention to form an intention. Just try to imagine it - if you have an intention A to form an intention B, you actually already have the intention B in your mind, so the intention A is a fiction. If we cannot have intentions to form an intention that makes any regress of intentions impossible, and thus all of our intentions emerge unintentionally in our minds. And then they determine our intentional actions.
What happens if we have divergent intentions at the same time (similar to divergent desires)? Isn't this the point where free will could choose one intention over another? Well, this choosing would have to be an intentional action, which would bring us to the problems described above. In general, our intentional action will be determined by the joint influence of all our intentions, and stronger intentions will weigh more heavily in the result than weaker ones.
In conclusion, intentionality is regarded as a necessary component of free will, but at the same time it rules out free will. That's why free will, as it is usually understood, is impossible.
However, an agent that does an intentional action can never be the ultimate controller of this action, because the action is determined by an intention (and possibly also by some influences affecting the agent). For example, if raising of my left arm is my intentional action, then it must be determined by my intention to raise my left arm. But how does the determination of an action by the agent's intention rule out the agent's ultimate control over the action?
To answer that question let us first look at what an intention is. Here we may help ourselves with some definitions in a dictionary, such as the Merriam-Webster dictionary (www.m-w.com). The first definition of the word "intention" in this dictionary is "a determination to act in a certain way", which indicates that an intention is A STATE OF MIND. Another relevant definition of this word is "what one intends to do or bring about", which indicates that an intention is a goal or purpose, AN IDEA one has in mind.
So A STATE OF MIND or AN IDEA determines an intentional action. Then for an agent to be in control of his intentional action it seems that he must determine his state of mind or the idea he has in mind, and through these he can determine the action. However, determining a state of mind or an idea in mind is an action too! It is an action of creating the state of mind or the idea and if THIS action is not intentional then obviously the agent is not in control of it and consequently not in control of the final action determined by the state of mind/idea. On the other hand, if the determining of the state of mind/idea is an intentional action, then it must be determined by an intention to create that state of mind/idea, and this intention is just another state of mind/idea. So we get an infinite regress of states of mind/ideas determining other states of mind/ideas, in which there is no place for an agent's ultimate control of his actions. Or assuming (as we usually do) that the agent's mind has a finite history, we arrive at the first state of the agent's mind or the first idea in his mind, which is not determined by his preceding states of mind/ideas, and therefore it is not intentional, he doesn't have control over it and consequently he doesn't have control of actions determined by it.
And moreover, it seems doubtful that it is possible to even have an intention to form an intention. Just try to imagine it - if you have an intention A to form an intention B, you actually already have the intention B in your mind, so the intention A is a fiction. If we cannot have intentions to form an intention that makes any regress of intentions impossible, and thus all of our intentions emerge unintentionally in our minds. And then they determine our intentional actions.
What happens if we have divergent intentions at the same time (similar to divergent desires)? Isn't this the point where free will could choose one intention over another? Well, this choosing would have to be an intentional action, which would bring us to the problems described above. In general, our intentional action will be determined by the joint influence of all our intentions, and stronger intentions will weigh more heavily in the result than weaker ones.
In conclusion, intentionality is regarded as a necessary component of free will, but at the same time it rules out free will. That's why free will, as it is usually understood, is impossible.