- We should refrain from believing falsehoods. (Premise)
- Whatever should be done, can be done. (Premise)
- If hard determinism is true, then whatever can be done, is done. (Premise)
- I believe that Free Will exists. (Premise)
- We can refrain from believing falsehoods. (From 1,2)
- If hard determinism is true, then we refrain from believing falsehoods. (From 3,5)
- If hard determinism is true, then Free Will exists. (From 4,6)
- Free Will exists. (7 implies that hard determinism is false)
Premise 1 shall examine in a moment.
Premise 2 is the "should implies can" principle, the idea that one cannot be expected to do anything which they are incapable of doing.
Premise 3 is, like 2, undeniably true. Determinism is the idea that things could only have gone as they have gone, and can only go in one way.
Premise 4 is something we'll have to take his word for. However, as it so happens I (on balance, it's far from something I'm certain of) also believe in free will, so we'll take it as given.
The remaining steps follow through naturally.
Premise 1, however, I believe, contains an assumption of free will. Why? Because if free will does not exist, then we have severe reason to doubt the existence of moral responsibility for our actions. If so, it is hard to see how we "should" do things. The claim that we "should" refrain from believing falsehoods therefore seems to contain the implicit assumption that we have free will. Huemer's argument therefore relies upon circular logic.
However, this does not make the argument useless. It provides a very strong case against the compatibility of determinism and moral responsibility.