There is an argument against determinism that is as old as time. You can find many different statements of it.

Here is one by the old Welsh theologian H. P. Owen:

Determinism is self-stultifying. If my mental processes are totally determined, I am totally determined either to accept or to reject determinism. But if the sole reason for my believing or not believing X is that I am causally determined to believe it I have no ground for holding that my judgment is true or false.

H. P. Owen, Christian Theism (Edinburgh, 1984), p. 118.

For a long time, I thought this was a strong argument. However, I now think it is mistaken.

The problem is that it introduces an assumption about determinism that is not actually part of determinism itself. The assumption is subtle. In fact, it is a single word. Notice the way it sneaks into Owen's argument:

If my mental processes are totally determined, I am totally determined either to accept or to reject determinism. But if the sole reason for my believing or not believing X is that I am causally determined to believe it I have no ground for holding that my judgment is true or false.

Not so fast! Determinism does not say that the sole reason you believe something is that you were determined to. It only says that everything is determined.

But you can be determined to believe something for good reasons.

Suppose Shakespeare, in writing a play, determines Hamlet to believe that A equals C because A equals B and B equals C. In this case, Hamlet's belief is well-founded. So, it is simply not the case that if determinism is true, Hamlet has no ground for holding that his judgment is true or false.

This example reveals the deeper problem with the argument to be the more general assumption that there can only be one reason for something:

1. Either Hamlet believes something because he has good reasons, or he believes it because he was determined to. It can't be both!

But why think that this is true? The word “because” has more than one sense:

1. The window shattered because its silicon and oxygen atoms were pulled beyond their elastic range.
2. The window shattered because God determined that its time had come.
3. The window shattered because I threw a rock at it.

Only a toddler would insist that these cannot all be true at the same time. But that is what this argument against determinism amounts to. “Hamlet only believes A equals C because he was determined to, not because he had good reasons!”

Why can't it be both? There is no reason why not.

Okay then: this is a bad argument.