There are people who argue that free will is an illusion. One notable proponent of this theory is Sam Harris. There are others who go further and say consciousness itself is an illusion.
Tom Campbell argues that consciouness and free will are intrinsically and inseparably combined; it is not possible to have one without the other. I'm not convinced this is necessarily the case. My experience is that this seems to be true: my consciousness and my ability to make decisions appear to be intertwined experientially. I can, however, imagine consciousness existing without free will; why is it not possible for a consciousness to exist that can be aware and observe and pay attention to existence and yet not be able to act or choose? In fact does consciousness even necessitate the ability to be aware of anything other than itself? The essence, and only necessary function of consciousness is to be conscious, that is to be conscious of its own existence as consciousness. Being able to be cognisant of anything outside of itself is an additional capability and being able to initiate expression or action is a further ability, and being able to choose between different acts is yet another additional capability.
So the argument that consciousness cannot be an illusion does not mean that free will cannot be an illusion. The evidence however seems overwhelming and conclusive that we do have free will. This is the underlying assumption of how we act and interact with each other and within society. So unless there was any evidence to the contrary, why even posit the idea that free will may be an illusion?
There are two sources of evidence against free will. One from scientific experiments, the other from spiritual experience. I will argue that neither are persuasive.
Susan Blackmore makes the argument that she, through meditation, has experienced herself to live without free will, as have a number of experienced enlightened spiritual masters. The fact that we can have this experience is not evidence that free will doesn't exist, it is simply evidence that some people, some of the time, can experience living without consciously engaging the faculty of free will. These experiences are exceptional and so that means that the vast majority of experiential and subjective evidence very strongly suggests free will does exist.
To say otherwise requires a judgment to be made on the relative realness of these two different experiences. It seems, logically, equally valid for someone to argue that the experience of not having free will is the illusory experience. It seems more reasonable to conclude nothing stronger than both experiences are possible, living whilst using free will and living without it; why relegate one of these experiences to the category of illusory?
To do this more evidence would be needed as to why the experience of having free will is not what we think it is. This is where the experiments come in.
Experiments have shown that brain activity can be recorded that can be used to predict the subjects actions and this brain activity occurs before the subject records themselves as being aware that they have made a conscious decision. Sam Harris and others argue this is evidence that our experience of making conscious decisions is not what is actually happening, as before this, the decision has already been made and can be recorded in our brain activity. This leads them to also argue that our decisions are not freely chosen but are determined by physical processes.
Firstly, let me say these experiments are very interesting, surprising, illuminating and worthy of discussion and further exploration, but they are not evidence that free will doesn't exist. What they do seem to show is that what we may have thought of as being the action of free will, the moment of making a decision in our conscious thoughts, is not the whole story of free will decision making.
Predictability does not equal determinism. In the cited experiments it is said that the freely chosen action - for example the raising of your left or right hand - can be predicted up to 5 or even 7 seconds before the conscious decision is made. The problem with this being equated to determinism is that we know that we can make instant decisions to override existing propensities.
Correlation does not equal causation. Science can provide no mechanism for how brain activity could cause consciousness nor conscious decision making. These experiments show correlation not cause. The argument for a causal link is made purely on the basis that that would fit in with the theory of materialistic determinism. Similar experiments show pre-sentience*: our bodies react to a randomly selected emotionally charged image before it is seen by the subject. It would be as reasonable to conclude that the emotional reaction in the body is determining which image gets selected as it would be to argue that brain activity is determining the conscious choice to be made. In fact, in both types of experiment we do not know what the underlying causal processes might be. The materialist will assume a causal link in the free will experiment but not in the pre-sentient experiment because that what fits their belief system.
http://psi-encyclopedia.spr.ac.uk/articles/presentiment#Presentiment_Meta-Analyses_