As science and philosophy explained ever more of the natural world in the Modern Era, there arose the philosophical idea of strict determinism. Strict determinism, as often presented, includes both metaphysical and epistemic aspects. In regards to the metaphysics, it is the view that each event follows from previous events by necessity. In negative terms, it is a denial of both chance and free will. A religious variant on this is predestination, which is the notion that all events are planned and set by a supernatural agency (typically God). The epistemic aspect is grounded in the metaphysics: if each event follows from other events by necessity, if someone knew all the relevant facts about the state of a system at a time and had enough intellectual capabilities, she could correctly predict the future of that system. Philosophers and scientists who are metaphysical determinists typically claim that the world seems undetermined to us because of our epistemic failings. In short, we believe in choice or chance because we are unable to always predict what will occur. But, for the determinist, this is a matter of ignorance and not metaphysics. For those who believe in choice or chance, our inability to predict is taken as being the result of a universe in which choice or chance is real. That is, we cannot always predict because the metaphysical nature of the universe is such that it is unpredictable. Because of choice or chance, what follows from one event is not a matter of necessity.
One rather obvious problem for choosing between determinism and its alternatives is that given our limited epistemic abilities, a deterministic universe seems the same to us as a non-deterministic universe. If the universe is deterministic, our limited epistemic abilities mean that we often make predictions that turn out to be wrong. If the universe is not deterministic, our limited epistemic abilities and the non-deterministic nature of the universe mean that we often make predictions that are in error. As such, the fact that we make prediction errors is consistent with deterministic and non-deterministic universes.
It can be argued that as we get better and better at predicting we will be able to get a better picture of the nature of the universe. However, until we reach a state of omniscience we will not know whether our errors are purely epistemic (events are unpredictable because we are not perfect predictors) or are the result of metaphysics (that is, the events are unpredictable because of choice or chance).
Interestingly, one feature of reality that often leads thinkers to reject strict determinism is what could be called chaos. To use a concrete example, consider the motion of the planets in our solar system. In the past, the motion of the planets was presented as a sign of the order of the universe—a clockwork solar system in God’s clockwork universe. While the planets might seem to move like clockwork, Newton realized that the gravity of the planets affected each other but also realized that calculating the interactions was beyond his ability. In the face of problems in his physics, Newton famously used God to fill in the gaps. With the development of powerful computers, scientists have been able to model the movements of the planets and the generally accepted view is that they are not parts of deterministic divine clock. To be less poetical, the view is that chaos seems to be a factor. For example, some scientists believe that the gas giant Jupiter’s gravity might change Mercury’s gravity enough that it collides with Venus or Earth. This certainly suggests that the solar system is not an orderly clockwork machine of perfect order. Because of this sort of thing (which occurs at all levels in the world) some thinkers take the universe to include chaos and infer from the lack of perfect order that strict determinism is false. While this is certainly tempting, the inference is not as solid as some might think.
It is, of course, reasonable to infer that the universe lacks a strict and eternal order from such things as the chaotic behavior of the planets. However, strict determinism is not the same thing as strict order. Strict order is a metaphysical notion that a system will work in the same way, without any variation or change, for as long as it exists. The idea of an eternally ordered clockwork universe is an excellent example of this sort of system: it works like a perfect clock, each part relentlessly following its path without deviation. While a deterministic system would certainly be consistent with such an orderly system, determinism is not the same thing as strict order. After all, to accept determinism is to accept that each event follows by necessity from previous events. This is consistent with a system that changes over time and changes in ways that seem chaotic.
Returning to the example of the solar system, suppose that Jupiter’s gravity will cause Mercury’s orbit to change enough so that it hits the earth. This is entirely consistent with that event being necessarily determined by past events such that things could not have been different. To use an analogy, it is like a clockwork machine built with a defect that will inevitably break the machine. Things cannot be otherwise, yet to those ignorant of the defect, the machine will seem to fall into chaos. However, if one knew the defect and had the capacity to process the data, then this breakdown would be completely predictable. To use another analogy, it is like scripted performance of madness by an actor: it might seem chaotic, but the script determines it. That is, it merely seems chaotic because of our ignorance. As such, the appearance of chaos does not disprove strict determinism because determinism is not the same thing as unchanging.