Thoughts are really something physical. If they are physical, they have a physical cause. All physical events are automatic events. So thoughts are automatic events. Determinism.
A process can only be described a deterministic if you can say that for a given set of conditions a given outcome will arise. It doesn't make any sense to call a process deterministic just because the outcome had a set of physical causes. If we accepted this latter definition then all physical processes would be defined as deterministic - and that isn't the case.
Evolution is a good example of a non-deterministic process. If evolution were deterministic, then given current conditions I could determine the exact range of animals that would evolve from my pet dog over the next 100,000 years. I can't do that because there are at least two things I can never predict: the random mutations that will occur within her descendants' genes ; and the precise environmental conditions they will encounter over the years.
Similarly, the process by which you acquired your beliefs is non-deterministic. Given a complete knowledge of your physical state at age 20, I couldn't predict your beliefs at age 35 because I can't know what experiences you will have in the intervening years, and I can't know what choices you'll make.
A previous poster gave the movement of a quantum particle as an example of a non-deterministic process. You dismissed this example with the argument that the final condition of the quantum particle must have had physical causes, therefore the process must have been deterministic. I hope you can now see the problem with your argument.
(Just in case you can't!). The movement of a quantum particle is non-deterministic because, given an initial set of conditions, we can only give a probability of certain outcomes arising - we can never predict with certainty what the final outcome will be.
The true mystery of the world is the visible, not the invisible