We know cause and effect exist in the universe. We can use this to gain control of a lot of things in our world: for instance, when I push the letter “A” on my touchscreen, the letter “A” appears on my screen due to these cause/effect systems we have set up.

However, we know that the universe is not entirely describable via cause and effect. Heisenberg’s Uncertainty Principle precludes us from fully observing all aspects of a quantum system, Gödel’s Incompleteness Theorem shows that our knowledge of the universe will always be incomplete, and Schrödinger's Cat shows us the absurdity of trying to make concrete claims about observed phenomena using probabilistic models.

If the universe was purely deterministic and we were theoretically able to gain all knowledge of it, there would not be free will. But this is not the universe we live in. The universe we live in is one where:

• Probability exists, so we cannot fully predict the future state of events even with perfect knowledge (i.e. Schrödinger can never gain enough knowledge to predict whether the cat is alive or dead before opening the box, because the event is fully probabilistic.)
• There are aspects of the universe we cannot fully observe, because by focusing on some aspects we must filter out others, so observation will never be fully reliable.
• Regardless of how much we learn, there will always be knowledge we cannot fully categorize.

If events in the universe were fully deterministic, then free will would be an illusion, because everything could be traced to an earlier set of causes, and decisions would not actually exist. If events in the universe were random, free will would have no meaning, because decisions would be arbitrary.

But we live in a universe where things are not immutable, but things are not equally likely. I can roll a fair set of dice for randomness, or I can weight them to create an uneven probability, or I can select a number to eliminate probability altogether. And we all make decisions with limited observations using incomplete knowledge that will only have a partial effect to affect the probabilities of future events. And that means we shape events without controlling them, so all of our decisions have meaning. We can also tell objectively based on observations that some decisions are better than others, while at the same time conceding that no decision is 100% objectively right or wrong.

And that, my friend, is free will.