Determinism is a view about the world, based on the belief that what is happening everywhere is predetermined (established, fixed, determined) and therefore necessary, which leads to the conclusion that the events could not have happened otherwise than they happened.
The intuitive idea of determinism can be summed up by saying that the world is like a movie: the image that is now projected represents the present. Those parts of the movie that have already been played are the past, and those that haven’t yet been presented are the future.
In the movie the future coexists with the past; and the future is fixed, exactly the same, as the past. Although the spectator may not know the future, each future, without exception, could in principle be certainly known. In fact, the future is even known by the film producer – The Creator of the World.
The idea of determinism is of religious origin, although there are great religions that believe in nondeterminism – the doctrine claiming that at least some events are not fixed in advance.
Religious determinism is linked to the ideas of divine almighty, the power to fully determine the future, and divine omniscience, which implies that the future is already known to God, being fixed in advance.
Besides religious determinism, there is a form of the deterministic doctrine which I call it “scientific”. From a historical point of view, the idea of “scientific” determinism can be seen as a result of replacing the idea of God with the idea of nature, and the idea of divine law with the law of nature.
Nature or perhaps the law of nature is omnipotent and omniscient. They fix everything in advance. In contrast to God, whose will is hidden from sight and can be known only by revelation, the laws of nature can be discovered by man’s reason with the help of his experience. And if we know the laws of nature, then we can foresee the future based on present day data by purely rational methods.
Characteristic of all forms of deterministic doctrine is that every event in the world is predetermined: if at least one future event is not predetermined then determinism must be rejected, nondeterminism being true.
from Karl Popper, The Open Universe