Hume’s Predictability Argument

1. (Premise) If actions are not determined by one’s character and external circumstances, then they are not predictable.
2. (Premise) Actions are predictable.
3. Therefore, actions are determined by one’s character and external circumstances.

Hume’s No Problem Argument

1. (Premise) That event A is always in fact followed by event B and that B is expected to follow A is compatible with event B being freely caused.
2. (Premise) That event A causally necessitates event B is nothing more than that A is always in fact followed by B and that B is expected to follow A.
3. therefore, that event A causally necessitates event B is compatible with event B being freely caused.

A Humean Responsibility Argument

“But to proceed in this reconciling project with regard to the question of liberty and necessity; the most contentious question of metaphysics, the most contentious science; it will not require many words to prove, that all mankind have ever agreed in the doctrine of liberty as well as in that of necessity, and that the whole dispute, in this respect also, has been hitherto merely verbal. For what is meant by liberty, when applied to voluntary actions? We cannot surely mean that actions have so little connexion with motives, inclinations, and circumstances, that one does not follow with a certain degree of uniformity from the other, and that one affords no inference by which we can conclude the existence of the other. For these are plain and acknowledged matters of fact. By liberty, then, we can only mean a power of acting or not acting, according to the determinations of the will; this is, if we choose to remain at rest, we may; if we choose to move, we also may. Now this hypothetical liberty is universally allowed to belong to every one who is not a prisoner and in chains. Here, then, is no subject of dispute.”

1. (Premise) If an action is not determined by one’s character and external circumstances, then the action is a matter of chance.
2. (Premise) If an action is a matter of chance, then one is not responsible for it.
3. Therefore, if one is responsible for an action, then the action is determined by one’s character and external circumstances.

Frankfurt’s Argument

1. (Premise for reductio) Necessarily, if you freely decided to do A, you could have not decided to do A.
2. (Premise) You can freely decide to do A while a neurosurgeon is scanning you, even though there is a sign S such that if S occurs, you for sure will decide to do A, and if S does not occur, the neurosurgeon for sure will make you decide to do A.
3. (Premise) If the neurosurgeon is scanning you as described in (1), then you cannot not decide to do A.
4. Therefore, you cannot be free in the neurosurgeon scenario. (By (1) and (3))
5. But you can be free in the neurosurgeon scenario. (By (2))