What do you think: does fate determine our lives or does our free will allow us to make our own choices? It's a question so old it practically swam in the primordial ooze of questions, and the characters of Foundation have to deal with this one a lot. Psychohistory can predict the future for large numbers of people, thanks to human behavior being predictable. But it's not good at determining the future of an individual. So, it seems like you might have free will as an individual, but not as a member of society. But do the decisions you make really matter? If the course of humanity's future has already been mapped out, then what purpose do you, as an individual, even have? This could get interesting.
Fate doesn't actually factor into Seldon's Plan, because the Plan focuses on statistics alone. So, he is less a prophet and more a card-counter. A really, really good card-counter.
In Seldon's plan, people are subjected to history, rather than being the ones who enact history.