Laplace’s Gremlin and Irreducibility

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.2

The curve described by a simple molecule of air or vapour is regulated in a manner just as certain as the planetary orbits; the only difference between them is that which comes from our ignorance.3

Stephen Hawking’s book1, A Brief History of Time, is one of the most influential popular physics books to have been written. But it may also be the source of a confusion about the meaning of another great physicist’s idea, Laplace’s demon.

The demon appears in an 1814 paper. Here is a 1902 English translation.

Laplace’s demon working away on the natural laws, data and computation necessary for deterministic prediction

Hawking