Determinism is the claim that the present state of the universe is determined by its past states, perhaps in conjunction with some other element such as causal laws. Some religious forms of determinism assert that events are determined by the will of God, or by other abstract concepts such as 'fate'. Generally, the variety of determinism relevant to philosophical and scientific inquiry is causal determinism. Causal determinism is the claim that, given the causal laws and the actual history of the universe, the present state of the universe could not have turned out differently. Determinism is incompatible with libertarian accounts of free will, though it may not be incompatible with free will generally. The claim that determinism and free will are compatible is called compatibilism; the claim that they are incompatible is (unsurprisingly) incompatibilism.
 Causal Determinism
Consider throwing a die, for example. We might see this as a way to determine a random result (between 1 and 6). However, the die is bound by the laws of physics, and, when it is thrown from a certain height and angle, with a certain force and speed, it will land on a certain edge or corner, and roll a certain number of times as it slows down and stops. Thus the result is determined as we roll it, although we may be unable to calculate all the factors involved and predict the outcome.
Similarly, decisions are made in the brain, which is also subject to the laws of physics. In general, our present brain states are causally connected to past brain states and may be causally determined by them (together with outside stimuli and the relevant causal laws). Even on a more abstract level, our choices are determined by the immediate situation, consequences we can predict, our past experiences of similar situations, and our own psychology and personality, which are also shaped by past experiences. Determinism does not entail predictability, since we may not be able to ascertain all of the causally relevant features of the world or the relevant causal laws.
Incompatibilists maintain that determinism entails that we are not the autonomous individuals capable of free choice which we think we are, but are essentially cogs in the machinery of the universe. Compatibilists, in contrast, think determinism is consistent with free will. Daniel Dennett, for instance, has argued that free will is a matter of control over our circumstances, and that this control is consistent with determinism but difficult to square with indeterminism. Incompatibilists can, broadly speaking, be divided into groups. Libertarians maintain that determinism is inconsistent with free will, and since we have free will it follows that determinism is false. Hard determinists also maintain that determinism and free will are inconsistent, and determinism is true; hence we lack free will. Hard determinists accept that we seem to experience free choice in making decisions and offer various explanations for these appearances.
Since it is difficult to justify the claim that reality is entirely chaotic and random, most believers in indeterminism promote the ideas that not all events have causes, or that all events have causes of a type falling short of strict determinism.
 Varieties of determinism
Determinism can come in a number of flavors, though it is generally seen as a Bad ThingTM in academic scholarship and anything with the word "determinism" attached to it is bound to be a term of abuse.
- Biological determinism (also genetic determinism, "biologism"): Anatomy is destiny, per Freud.
- Cultural determinism: People are entirely the products of culture.
- Economic determinism (also "economism"): Economics shapes politics and the course of history. This was the official line of the orthodox Marxism espoused by the Second International, the precursor of Leninism.
- Environmental determinism: Natural resources and geography determine the course of history, though the term is sometimes used synonymously with cultural determinism.
- Historical determinism: Any teleological formulation of history. Other forms of determinism are often subsets of historical determinism.
- Psychological determinism (also "psychologism"): Psychology determines everything. Often associated with offshoots of Freudian theories such as psychohistory.
- Technological determinism: Technology determines everything. Its most popular form is probably futurism.