[lawtguy]

This world view is a very 19th century clockwork universe one: if we know all the initial conditions in sufficient detail, we can predict the outcome. There are two problems with this: at a fundamental level, quantum mechanics says you can't know the initial conditions with infinite precision[0]. Second, even at a classical physics level non-linear systems with very similar starting points can have very different final states. As a practical matter that means even if the rules are deterministic, you cannot predict with 100% accuracy what the outcome will be.

So we have a complex non-linear system (your genes) and a very very complex non-linear system (the environment) that make a very complex non-linear system (your brain). Because we have all of these complex non-linear system interacting together, we cannot determine what will happen in any practical sense. We can probably make some observations that will generally hold about some decisions a human brain will make (e.g. it's been a few hours, it will be hungry and decide to eat), but these are more probabilistic rather than 100% concrete deterministic rules.

0: https://en.wikipedia.org/wiki/Bell%27s_theorem