Suppose as is very plausible (except for dubious interpretations of "can do otherwise") that you know that
- Determinism implies that you cannot ever do otherwise than you in fact do.
- You will in fact do A,
- Determinism holds.
- You cannot do otherwise than A.
This is interesting. For while determinism does not by itself guarantee the possibility of knowledge of how you are determined to act, it turns out that with a bit of induction and reflection, if you know determinism to be true, you are in a position to know what you are determined to do.
It is also plausible that:
- When you know you cannot do otherwise than A, then you are not freely choosing A.
But now we get the interesting conclusion that if you know determinism to be true, that knowledge could very well undercut some of our freedom. For it could boost knowledge of what we will in fact do to knowledge of what we will have to do.
Objection 1: Knowledge of what we will in fact do does take away freedom, so knowing that we will have to do it doesn't take away any freedom that wouldn't already have been taken away.
Response: I know I will eat lunch today, but that doesn't take away my freedom.
Objection 2: Claim (5) is no more plausible than the disjunction of the following two principles:
- When you have a belief with knowledge-level justification that you cannot do otherwise than A and you think you know that you cannot do otherwise than A, then you are not freely choosing to do A
- When you cannot do otherwise than A, then you are not freely choosing to do A.
Response: Maybe. But maybe the right way to reason is this. Clearly (5) is true. Now, there are two initially plausible explanations for (5), namely (6) and (7). Since (6) is false, that leaves (7). So we have an inference to best explanation from (6) to (7). And so, even though previously I was only arguing for the interesting conclusion that knowledge of determinism could take away some freedom, we have arrived at an argument for incompatibilism. The argument starts with (5), concludes to (7) by inference to best explanation, then adds (1), and concludes that freedom is incompatible with determinism.