Determinism is the idea that everything that happens, including all human actions, is completely determined by prior events. There is only one possible future, and it is completely predictable in principle, most famously by Laplace's Supreme Intelligent Demon, assuming perfect knowledge of the positions and velocities of all the atoms in the void. More strictly, determinism should be distinguished from pre-determinism, the idea that the entire past (as well as the future) was determined at the origin of the universe. Determinism is sometimes confused with causality, the idea that all events have causes. But some events may be undetermined by prior events. Such an event is indeterminate, sometimes known as a "causa sui" or self-caused event. But it may in turn be the cause for following events that would therefore not be predictable from conditions before the uncaused event. We call this "soft" causality. Events are caused, but they are not always predictable or determined. Uncaused events are said to break the "causal chain" of events back to a primordial cause or "unmoved mover." Aristotle's "accidents" and Epicurus' "swerve" are such uncaused causes. Although there is only one basic form of indeterminism, there are many determinisms, depending on what pre-conditions are considered to be determinative of an event or action. We identify more than a dozen distinguishable determinisms below.
There is only one irreducible freedom, based on a genuine randomness that provides for a world with breaks in the causal chain. Quantum mechanics is the fundamental source for indeterminacy and unpredictability in the physical, biological, and human worlds. It generates the Agenda for our Micro Mind in the Cogito model for Free Will. Philosophers and religious thinkers become quite perplexed when considering the basic conflict between such an irreducible freedom and their own particular determinism. Because interpretations of quantum mechanics are difficult even for physicists, most philosophers dodge the issue and declare themselves agnostic on the truth of determinism or indeterminism. Even some philosophers who accept the idea of human freedom are uncomfortable with the randomness implicit in quantum mechanics and the uncertainty principle. True chance is problematic, even for many scientists, including those like Planck and Einstein, who discovered the quantum world. And for traditional philosophers in a religious tradition, chance has been thought to be an atheistic idea for millenia, since it denies God's foreknowledge. Chance, they say, is only the epistemic problem of human ignorance.
Behavioral Determinism assumes that our actions are reflex reactions developed in us by environmental conditioning. This is the Nurture side of the famous Nature/Nurture debate - note that both are determinisms. This view was developed to an extreme by B. F. Skinner. Biological Determinism finds causes for our actions in our genetic makeup. This is the Nature side of the Nature/Nurture debate - both sides are determinisms. Causal Determinism finds that every event has an antecedent cause in the infinite causal chain going back to Aristotle's Prime Mover. There is nothing uncaused or self-caused (causa sui). Galen Strawson holds this view. Fatalism is the simple idea that everything is fated to happen, so that humans have no control over their future. Notice that fate has arbitrary power and need not follow any causal or otherwise deterministic laws. It can include the miracles of omnipotent gods. Historical Determinism is the dialectical idealism of Hegel or the dialectical materialism of Marx that are assumed to govern the course of history. Logical Determinism reasons that a statement about a future event happening is either true or it is not. If the statement is true, logical certainty necessitates the event (cf. Aristotle's Sea Battle). If the statement is not true, the event can not possibly happen. For logic. the truth is outside of time, like the foreknowledge of God. Linguistic Determinism claims that our language determines (at least limits) the things we can think and say and thus know. The Sapir-Whorf hypothesis claims that speech patterns in a language community are highly predictable. Mechanical Determinism explains man as a machine. If Newton's Laws of Classical Mechanics govern the workings of the planets, stars, and galaxies, surely they govern man the same way. Necessitarianism is a variation of logical and causal determinism that claims everything is simply necessary. Physical Determinism extends the laws of physics to every atom in the human mind and assumes the mind will someday be perfectly predictable once enough measurements are made. Psychological Determinism is the idea that our actions must be determined by the best possible reason or our greatest desire. Otherwise, our acts would be irrational.
Religious Determinism is the consequence of the presumed omniscience of God. God has foreknowledge of all events. All times are equally present to the eye of God (Aquinas' totem simul). Note the multiple logical inconsistencies in the idea of an omnipotent, omniscient, benevolent God. If God knows the future, he obviously lacks the power to change it. And benevolence leads to the problem of evil. Spatio-temporal Determinism is the view of special relativity. The "block universe" of Hermann Minkowski and Albert Einstein assumes that time is simply a fourth dimension that already exixts, just like the spatial dimensions. The one possible future is already out there up ahead of where we are now, just like the city blocks to our left and right. J. J. C. Smart is a philosopher who holds this view. He calls himself "somewhat of a fatalist." Compatibilism is the idea that Free Will is compatible with Determinism. Compatibilists believe that as long as our Mind is one cause in the causal chain that we can be responsible for our actions, which is reasonable. But they think every cause, including our decisions, are pre-determined. Compatibilists are Determinists. Some of these determinisms (behavioral, biological, historical-economic, language, psychological, and religious) have modest to significant evidence that they do limit human freedom. But others are merely dogmas of determinism, believed for the simple reason that they eliminate random chance in the universe. Chance is anathema to most philosophers.
Honderich: "Determinism is usually the thesis that all our mental states and acts, including choices and decisions, and all our actions are effects necessitated by preceding causes." Oxford Companion to Philosophy, p.293.