(Note: this question is not about predictability.)

I consider Laplace´s idea of the demon: “ 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.”

The statement was made in 1814, and I wonder what “forces” Laplace could contemplate. I assume the electrical forces and the atoms were only known as a phenomenon without too much quantitative context.

I speculate that his understanding of gravity perhaps was a model for all other “forces” that might come to play, that they might in principle be understood in detail in the (his) future. In any case it seems that his demon only had very sketchy tools to work with at the time.

What is your understanding of his thoughts about these forces and the, as I speculate, missing steps to a more complete model for his understanding of predictability?

  • You have to consider Laplace's treatise Méchanique Céleste five volumes, from 1799 and on. I do not think that he "had very sketchy tools to work with at the time". Nov 24, 2016 at 14:53
  • But yes: in it he considered gravitation. Nov 24, 2016 at 14:54
  • @Mauro Allegranza I am mainly worried about electicity, not gravitation. Nov 24, 2016 at 15:04
  • I think that Laplace's treatise on celestial mech does not consider it... Nov 24, 2016 at 15:12
  • 1
    Ampère published his work: Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l’experience (Memoir on the Mathematical Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience) in 1827: it coined the name of his new science, electrodynamics. Thus, the modern mathematical treatment of electricity is later than Laplace. Nov 25, 2016 at 9:39

2 Answers 2


The model that Laplace was probably contemplating was Newtonian Mechanics; this theory is deterministic, so he was simply dramatising or even melo-dramatising via his demon the notion of determinism implicit in this theory.

As the character in this drama is a demon rather than an angel suggests that he supposes that there is something not quite right here, after all we experience the world as a mixture of neccessity and freedom.

  • According to Wikipedia (Laplace´s demon) he didn´t use the word demon himself. Nevertheless, this was a time where religion and philosophy was interwoven and he did not use God, the “hypothesis” of which Laplace had no need. Bishop Berkeley had included God in his speculations some time earlier, and the lack of a divine actor in a cosmic theory is evident so I expect a “demon” is very much in character, as viewed by his contemporaries. Dec 2, 2016 at 15:05
  • @jensen: interesting and surprising detail - rather like Newtons apple I suppose; it does rather make one think who introduced it! Dec 8, 2016 at 3:21

Since Laplace mentions "intellect" in there, it is much more a collective knowledge he means (rather than a specific physics force). In his time it was very straightforward to address an "Unknown" (as expression) as a whole to be responsible for a bulk of effects. By the advance of science, we could organize into cascaded structure the revealed phenomenas, and easily branching it to separate areas if proves to be multidisciplinary.

Addressing an intellect, being aware of all materia and all effects in perceptable universe, sounds to be much more looking for an entity owning this skill. I think it is a safe assumption that this entity would be called as god (as that is a popular topic expected from science, to prove the existence), or should the word "demon" feel more comfortable, then let's roll with that.

Part 1/2
The first half of the thesis, that all materia and effect on them can be described in one formula is a matter of scale of the scope. The depth and granulation of the study calls for more or less parameters, so twisting this part to simplification, going to extremely high and detailless level, it will be possible to compile one single formula. On the contrary I think it is not what he was looking for, but the further going into details, it will call for specification. Maybe it would sound better to say "for every single smallest possible thing, there is a finite but ultimately long formula to determine, on what conditions will that thing get into a specific other state". So it is not a single formula for all elements, but as many formulas, as many studied states is to be discussed for a single element.

Part 2/2
The second part of the thesis goes for, if that skill is possible to own, Laplace expects the entity to be able to conclude all possible past events and future outcomes. In order to agree with the statement, I would request the specific restriction to discuss "universe" as a closed environment, not becoming subject to external events, so all mechanics to be analyzed are inclusive.

To be honest I think the universe is never just as big as far we describe it at the moment of scientific level. Like Earth was believed to be flat, then Sun was believed to go around the Earth, then Ether was believed to exist over the atmosphere, etc. We have current conlusions, but I'm sure we will have more later on. Universe is a superlative, but maybe it will become a name for a border when we recognize something beyond it. So I would not use such restriction, as "the universe can't be subject to external event". So I mark the second part of the thesis unachievable for longer time, than for the actual moment.

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