Laplace’s Demon

Laplace strongly believed in causal determinism, which is expressed in the following quotation from the introduction to the Essai:

We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.

This intellect is often referred to as Laplace’s demon. Note, however, that the description of the hypothetical intellect described above by Laplace as a demon does not come from Laplace, but from later biographers: Laplace saw himself as a scientist; and while hoping that humanity would progress to a better scientific understanding of the world, he recognized that, if and when such an understanding were eventually completed, a tremendous calculating power would still be needed to compute it all in a single instant. While Laplace saw foremost practical problems for mankind to reach this ultimate stage of knowledge and computation, later interpretations of quantum mechanics, which were adopted by philosophers defending the existence of free will, also leave the theoretical possibility of such an “intellect” contested.

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