Determinism is the philosophical proposition that every event, decision and action is causally determined by an unbroken chain of prior occurrences. This does not necessarily mean that humans have no influence on the future and its events (a position more correctly known as Fatalism), but that the level to which humans have influence over their future is itself dependent on present and past. Taken to its logical extreme, Determinism would argue that the initial Big Bang triggered every single action, and possibly mental thought, through a system of cause and effect.

Another variation arises from the idea of Deism, which holds that the universe has been deterministic since Creation, but ascribes the Creation itself to a metaphysical God or first cause outside of the chain of determinism.

Some hold that if Determinism were true, it would negate human morals and ethics. Some, however, argue that, through an extended period of social development, a confluence of events could have formed to generate the very idea of morals and ethics in our minds (a sort of chicken and egg situation).

Determinism can be interpreted in two main way:

• Incompatibilism is the belief that Free Will and Determinism are logically incompatible categories and therefore mutually exclusive. This could include believing that Determinism is the reality, and therefore Free Will is an illusion (known as Hard Determinism); or that Free Will is true, and therefore Determinism is not (known as Libertarianism); or even that neither Determinism nor Free Will is true (known as Pessimistic Incompatibilism).
• Compatibilism is the belief that Free Will and Determinism can be compatible ideas, and that it is possible to believe both without being logically inconsistent. By this definition, Free Will is not the ability to choose as an agent independent of prior cause, but as an agent who is not forced to make a certain choice. This leads to the position of Soft Determinism, proposed by the American Pragmatist William James on the grounds that thorough-going, or Hard, Determinism leads either to a bleak pessimism or to a degenerate subjectivism in moral judgment

In Buddhism, there is a theory called Dependent Origination (or Dependent Arising), which is similar to the Western concept of Determinism. Roughly speaking, it states that phenomena arise together in a mutually interdependent web of cause and effect, and that every phenomenon is conditioned by, and depends on, every other phenomena.

According to the ancient Chinese "Yi Jing" (or "I Ching", the "Book of Changes"), a kind of divine will sets the fundamental rules for the working out of the probabilities on which the universe operates, although human wills are also a factor in the ways in which we can deal with the real world situations we encounter.

In the West, the Ancient Greek atomists Leucippus and Democritus were the first to anticipate Determinism when they theorized that all processes in the world were due to the mechanical interplay of atoms.

With the advent of Newtonian physics, in the 17th Century, which depicts the physical matter of the universe as operating according to a set of fixed, knowable laws, it began to appear that, once the initial conditions of the universe have been established, then the rest of the history of the universe follows inevitably, (rather like billiard balls moving and striking each other in predictable ways to produce predictable results). Any uncertainty was always a term that applied to the accuracy of human knowledge about causes and effects, and not to the causes and effects themselves.

Since the beginning of the 20th Century, quantum mechanics has revealed previously concealed aspects of events, and Newtonian physics has been shown to be merely an approximation to the reality of quantum mechanics. At atomic scales, for instance, the paths of objects can only be predicted in a probabilistic way. Some argue that quantum mechanics is still essentially deterministic; some argue that it just has the appearance of being deterministic; some that quantum mechanics negates completely the determinism of classical Newtonian mechanics.

• Causal Determinism (or Nomological Determinism) is the belief that future events are necessitated by past and present events combined with the laws of nature. Thus, all events have a cause and effect and the precise combination of events at a particular time results in a particular outcome.
• Logical Determinism is the notion that all propositions (i.e. assertions or declarative sentences), whether about the past, present or future, are either true or false. The question then arises as to how choices can be free, given that what one does in the future is already determined as true or false in the present.
• Environmental Determinism (or Climatic or Geographical Determinism) is the view that the physical environment, rather than social conditions, determines culture.
• Biological Determinism is the idea that all behaviour, belief and desire is fixed by our genetic endowment and make-up and cannot be changed.
• Theological Determinism is the belief that there is a God who determines all that humans will do, either by knowing their actions in advance (via some form of omniscience) or by decreeing their actions in advance.
• Emergentism (or Generativism) argues that free will does not exist, although an illusion of Free Will is experienced due to the generation of apparently infinite variations in behaviour from the interaction of a finite (and deterministic) set of rules and parameters. Thus the unpredictability of the emerging behaviour which we see in daily life actually stems from complex, but entirely deterministic, processes.