Dennett has an interesting, but invalid, argument for the compatibility of freedom & determinism. But I think I've come up with a modification which solves the problem.
Dennett's argument is that no-one knows whether determinism is true, but (according to incompatibalists) determinism implies not-freedom, which then implies not-morally-responsible. So, Dennet argues, the incompatibalist must think we never know whether people are morally responsible. But obviously we do, so the incompatibalist is wrong.
Despite it's initial plausibility, this argument is actually invalid, as our lecturer pointed out. if P implies Q, you CAN know Q without knowing P. For example, "If it rains tomorrow then 2+2=4" is true, but just because I don't know whether it rains tomorrow, does not mean that I don't know 2+2=4.
So it is logically possible to know whether we are morally responsible WITHOUT knowing whether determinism is true. So Dennett's argument falls apart.
But let's look at the logic more closely: If P implies Q, you can know Q without knowing P. That was Dennett's mistake. BUT, we can safely assert the reverse: If you know P implies Q, and you know P, then you DO also know Q. (This 'closure principle' is an important epistemologial principle. See my post on Nozick for more detail).
This suggests Dennett's argument might work if we run it in reverse:
1. No-one knows whether determinism is true.
2. We can (sometimes) know that S is a morally responsible agent.
3. If S is morally responsible then S is free.
4. (Incompatibalist premise) If S is free then determinism is false.
5. Therefore, we can (sometimes) know that determinism is false
I think the logic here is valid. If you can see any flaws, please comment & let me know!
These 5 statements are clearly inconsistent. But 1,2,3 all seem true, and 5 follows from 2,3 & 4 (via the closure principle, i.e. if X knows P, and knows that P implies Q, then X knows Q). Our best option seems to be to deny 4: That is, we should conclude that the incompatibalist is wrong.