Classical mechanics is built upon the concept of determinism. Determinism means that knowledge of the current state of a mechanical system completely determines its future (as well as its past). During the nineteenth century, determinism became a guiding principle for advancing our understanding of natural phenomena, from empirical evidence to first principles and natural laws. In order to formalise the concept of determinism, the French mathematician Pierre Simon Laplace postulated an intellect now referred to as Laplace's demon:

Laplace's demon has three properties: (i) exact knowledge of the laws of nature; (ii) complete knowledge of the state of the universe at a particular point in time (of course, Laplace was writing in the days before knowledge of quantum mechanics and relativity); and (iii) the ability to solve any form of mathematical equation exactly. Except for extremely rare cases, none of these three conditions is met in practice. First, mathematical models generally provide a much simplified representation of nature. In the words of the statistician George Box: “All models are wrong, some are useful”. Second, reality can only be assessed through measurements which are prone to measurement errors and which can only provide a very limited representation of the current state of nature. Third, most mathematical models cannot be solved analytically; we need to approximate them and then implement their solution on a computer, leading to further errors.