Open theism is the belief that god exists, but does not know the future outcomes of human choices. Instead, he finds out about them when they happen.
Open theism renders all human conclusions completely irrational.
A god who does not know what the future holds clearly did not design the future. And if he did not design the future, then there is no good reason to believe that it must behave in the way he wants it to behave.
Further, if god did not design the future, then time itself is not subject to him. It stands apart from god's oversight in some way, since the future is part of time and the future is not designed by god. This necessitates that time itself doesn't have to behave in the way god wants it to behave.
However, this means that all of our thoughts about the past, present, and future are without any rational warrant.
You see, all of our conclusions -- every single one of them -- are based on the belief that contradictions are false, and that they always have been and always will be. All of our conclusions are based on the idea that non-contradiction is invariant. But what could possibly rationally justify this belief?
If time is subject to God, and God is non-contradictory, then God Himself can rationally justify our belief in invariant non-contradiction. God is rational, He caused the facts to be facts, and He caused us to believe them and to base our conclusions upon them. This objectively rationally justifies our initial beliefs about non-contradiction. But if time is not subject to god, then god's nature is irrelevant as far as the nature of time goes. God's desires are irrelevant as far as the nature of time goes. God himself is irrelevant as far as the nature of time goes.
If there is nothing rational behind time, there can be nothing rational about beliefs about time.
Also note that non-contradiction is simply not a fact that can be "discovered." Instead, it is a first principle, standing foundationally beneath the process of discovery itself. You cannot discover anything at all without first presupposing non-contradiction. All things are meaningless without first presupposing non-contradiction. Therefore no one, including the god of open theism, could possibly "discover" that non-contradiction were true.
The god of open theism does not make time non-contradictory, and he cannot discover that time is non-contradictory. Time simly does not have to behave the way he wants it to. He therefore has no justification for claiming that it is non-contradictory. And if god has no justification for a belief in invariant non-contradiction, we sure don't.
Under open theism then, just like atheism, polytheism, and deism, all of our conclusions are without reason, since all of our conclusions assume invariant non-contradiction, and there can be no reason for assuming it.