According to libertarianism, 'freedom to choose otherwise' is a necessary requirement of free will. Generally speaking, when looking back on a choice, we would feel that we could have chosen a different option. However, this would not be the case if we had been physically forced to choose – for compatibilists, this is an important distinction. Hume held that we have free will as long as we have freedom from constraints – he argued that the 'prisoner in chains' is the greatest illustration of the unfree man.
Criticism: If determinism is correct, we can't choose otherwise, therefore free will and determinism are incompatible.
Hume's apparent dismissal of metaphysical freedom is concerning. Consider a ball rolling down a hill. Nothing is obstructing it, and nothing is forcing it to keep rolling – but we would not grant it free will. Although it is not physically constrained, it is constrained by the laws of nature – something is causing it to do what it is doing. Similarly, human actions seem to have been caused. I might act on a desire to kill somebody, and Hume would call this a free act – but according to physical determinism, my action is simply a part of the causal chain: it has been pre-determined. I could not have chosen otherwise, and am therefore not free!
The problem here is that while compatibilism accepts that a person would have chosen otherwise if they had chosen, in failing to refute physical determinism, it seems that they can't say that a person could have done otherwise. If a situation was repeated an infinite number of times, we would still have behaved the same way. So, it seems that 'freedom from constraint' is not enough – and therefore free will and determinism are not compatible.
In response to the above criticism, some compatibilists, including Hume, argue that even if our choices would be the same in the same situations, this does not undermine free will. Instead, it simply implies that we do things for reasons, and that similar stimulus leads to similar reasons. If a situation was repeated, and yet I behaved differently, surely this leads to randomness? For example: if I chose to eat an orange because I like oranges, I felt like something…