Free will (in the standard libertarian sense of the term) means the ability to choose from among different possible courses of action: when you have the ability to act freely, you can either perform or not perform some given action. That’s why free will is incompatible with determinism. This does not mean that free will is the same thing as random behavior, however. Free actions are ones for which you can be held responsible. But if your actions were the result of chance events – similar to the decay of a radioactive atom – then they would not be up to you, and you would be no more responsible for them than you are for the weather. They would be events that happened to you rather than events you brought about.
Free actions, then, are neither completely caused nor random. And herein lies the problem. For to lack complete causation is to be random. The decay of a radioactive atom is a chance event, not because it lacks any cause, but because it is not entirely determined by the prior state of things.
Suppose you are asked to pick one of two cards. You pick the right one. If you are free, you could have picked the left one instead. We can imagine the universe being rewound to the moment before you chose – so that everything is exactly the same, both within you and without you – only this time around you pick the left one. But in that case what explains the difference? Well, there could be something (S1) that led you to pick the right card one time and something else (S2) that led you to pick the left the other time. Nevertheless, everything is exactly the same both times, so what could explain S1 influencing you the first time and S2 the second?
Unless there is some difference to explain it, the choice must be random. And there is no difference! Therefore, the choice is random.
Some proponents of free will think they can escape this dilemma by claiming that we have reasons – which are neither caused nor random – for our choices. But again, if one time you accept reason R1 and the other time reason R2, what explains the difference? What makes you adopt one reason rather than the other? If under the exact same circumstances you can accept either R1 or R2, then which reason you end up with is itself ultimately random.
If there is only one possible course of action to you in any given situation, then your decisions are determined. If, on the other hand, you might either perform or not perform some action in any given situation, then your decisions are random. Either way, there is no free will. But of course, there are those who continue to believe in it while claiming that how it works is just a mystery. Anyone familiar with theological arguments has heard that before.
[Originally published at Debunking Christianity]