Determinism is the belief that all events are entirely determined by other, earlier events. Determinism carries the implication that the state of the world at one moment suffices to fix its state at a later moment. And because that later state fixes subsequent states, and so on, the conclusion is drawn that everything which ever happens in the future of the universe is completely determined by its present state. If the world is strictly deterministic, then all events are locked in a matrix of cause and effect. Thus, even free will is denied by determinism.
Contrary to determinism is indeterminism. We might say that an event happened by "pure chance" or "by accident" if it was not obviously determined by anything else. But are these cases of genuine indeterminism, or is it merely that the factors and forces are hidden from us? One expression of indeterminisn is known as Heisenberg's uncertainty principle. Roughly speaking, it states that all measurable quantities are subject to unpredictable fluctuations, and hence to uncertainty in their values. Thus stating, with quantum physics, that the universe is indeterministic at its most basic level. The question then arises as to why there are patterns, and why such mathematical schemes are possible.