Determinism is the philosophical doctrine that claims that all behavior results from preceding events or natural causes. It is result of the two preceding ideas: Materialism and Causality. For example, a proponent of determinism would say that the relentless laws of physics (as applied to neurons in the brain of the reader) caused the reader to read this article today.
Proponents of determinism sometimes claim that free will is an illusion, and that beings are no more able to control matter with their minds than any other soulless matter (such as a robot) can. One objection to determinism is that a universe in which people do not really make their own choices has no morality.
Determinism and free will are often, but not always, seen as mutually exclusive. The idea that they might be compatible (or that free will even requires determinism) is called Compatibilism.
It should be noted that even belief in a soul is not a solution, unless the soul itself behaves non-deterministically.
Much of a belief in determinism was inspired by Newtonian physics in which the universe was seen as a collection of billiard balls interacting according to the laws of physics. In this view, once the initial conditions of the universe were known, the behavior of the universe for all time would be determined. However, some (but not all) interpretations of quantum mechanics assert that the universe is non-deterministic, meaning that some events (such as when a radioactive atom decays) do not have immediate causes.
However, even non-deterministic interpretations of quantum mechanics do not necessarily allow for free will. (After all, a robot could be made to use quantum uncertainties to "decide what to do", but this would clearly not show that the robot had free will.) Although non-determistic interpretations of quantum mechanics allow for uncaused events, these events are statistically random and unaffected by human cognition. See also block time for an example of how the universe can be seen to be deterministic regardless of how the laws of physics operate within it.