Determinism has a number of different confusing usages, and it’s very easy to mix these usages up for free will debate. So what does it mean when someone uses the word “determinism” in regards to the free will topic? It simply means that every event is an effect of a cause. Every single one.
If we were to say that “The Universe is Deterministic”, what we’re saying is that, for every event that happens in the universe, some other event forces that outcome. In a deterministic universe, even at the teeniest tiniest Quantum scale, events that occur happen due to other existing factors. They are an effect of a cause.
What determinism doesn’t mean, and what people often confuse with determinism, is the ability to know all of the variables. In other words, determinism doesn’t imply predictability in the sense the we would be able to exactly know or predict an event. In chaos theory, trying to predict each variable to an exactness becomes impractical as one small mistake in a calculation can change the results in a drastic way. Such, however, is within the confines of an entirely deterministic (causal) setup.
In the world of the really small (Quantum Mechanics) we have a measurement problem. To “see” something at this scale (note: we don’t really “see” something at this scale in the literal sense of us visually seeing or observing it) we need to measure it. The problem when we get down to such a size, however, is that to measure a teeny tiny particle, we need to bounce something off of it that is the same size or almost the same size.
To analogize, think of measuring the temperature of a large tub of water with a thermometer. We place the thermometer into the water and due to the sheer volume of water, the thermometer is able to get a fairly accurate read. But imagine if that tub is now reduced down to a vile that is just a little larger than the thermometer itself. As soon as we place the thermometer into the vile, the temperature of the thermometer (the measuring device) gets dispersed into the vile of , changing the temperature of the water that is in the vile. We have a problem getting the temperature of the vile water due to the interaction with the measuring device itself.
A similar thing happens at the quantum scale. If we want to make a measurement of a particle, the measuring device we use basically entails sending other particles to interact with it. The interaction itself, however, is a causal event that takes place to the particle and hence changes it.
Also, in quantum mechanics, we have what is called the “uncertainty principle”. This means we simply cannot measure both the position and the momentum to an exactness at the same time. The more accurately we attempt to measure one, the more blurred becomes the other. Depending on ones “quantum interpretation” this could happen due to there being acausal events (in which case we would no longer be talking about a deterministic universe), or it can happen due to hidden variables (in a deterministic universe).
Another counter intuitive idea is that, in such a deterministic universe, the hidden variables at the Quantum scale would need to be “non-local”. This means “action at a distance”. An example of a non-local hidden variable interpretation of Quantum Mechanics is called Bohmian Mechanics or the de Broglie–Bohm theory. Non-local variables would make an entirely deterministic universe not entirely predictable, unless we were somehow able to see and understand the non-local causes that happen.
But it matters not. Determinism as used for the free will debate has literally nothing to do with us being able to know all of the variables (or to be able to predict with exactness). It has to do with the events having causes, and what it (logically) means that they do. I go over this thoroughly in the book Breaking the Free Will Illusion for the Betterment of Humankind.
For further information on Determinism and some of the confusions that arise with this term please read: