What is Determinism (a definition)
Determinism is the belief that everything that happens in the universe is determined by previous events and conditions. This includes human thought and actions, which implies that even our choices are a result of previous causes and conditions rather than decisions made of our own will.
Past, Present, and Future in a Deterministic Universe
Although there are practical limitations, if one presupposes a truly deterministic universe then having knowledge of all current states and all laws of nature, would allow us to have visibility into the past and the future. Pierre Simon Laplace, wrote about this in his 18th century essay. This thought experiment would later be known as Laplace’s demon:
We ought then to regard the present state of the universe as the effect of its anterior state and as the cause of the one which is to follow. Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective situation of the beings who compose it—an intelligence sufficiently vast to submit these data to analysis—it would embrace in the same formula the movements of the greatest bodies of the universe and those of the lightest atom; for it, nothing would be uncertain and the future, as the past, would be present to its eyes.
-- Pierre Simon Laplace, A Philosophical Essay on Probabilities
Determinism and Science (Quantum Mechanics)
Quantum mechanics, as currently defined, is non-deterministic. This has led many scientists to look for solutions that would allow it to become deterministic. Einstein, for example, was one supporter of a deterministic universe stated:
"God does not play dice with the universe" - Albert Einstein
Determinism, Free will, and Morality
If all events are determined by previous events, then our choices are also determined by previous events. This brings into question the nature of free will. Philosophers have struggled with this since it could undermine our sense of morality. In a deterministic world, it is difficult to judge others for their actions, since the actions taken were not anything they had a choice in taking.