The Principle of Sufficient Reason is a philosophical principle stipulating that everything must have a reason, cause, or ground. My question is: does quantum mechanics serve as evidence against it as I’ve seen some stipulate here? Do things happening purely randomly give us any indication that they are happening without reason?

First of all, I am having trouble understanding how quantum mechanics is purely random. For starters, given certain conditions, we can atleast assign probabilities to certain outcomes.

Secondly, why are things happening randomly any less likely to be caused than things happening in a way where each thing can be predicted? Even if we could for example predict the path of every particle for example, why does this imply that this is occurring for a reason? The law itself may exist without reason.

Secondly, it is not hard to imagine a universe that consists of some eternal entity that then “pokes” an electron in such a way where it then moves in unpredictable ways. This doesn’t imply that the electron’s movement is uncaused. And if something has a cause, it of course has a reason.

  • 1
    If a nucleus only has a certain probability of decaying and there is no hidden variable determining the exact time it will decay, then this seems to violate sufficient reason even if you can give a probability. There may be a reason why it has a probability of decaying, but there is no sufficient reason for why it decayed now instead of another time. Sep 3, 2023 at 2:20
  • How do you know that? Not being able to predict when it would have decayed does not imply it could have decayed at another time. You can’t rewind time to test that.
    – user62907
    Sep 3, 2023 at 2:21
  • Then there is a hidden variable--contrary to most versions of quantum mechanics. Sep 3, 2023 at 2:29
  • Why does that imply a hidden variable? There is no contradiction in not being able to predict something through any means and have it also be inevitable. Quantum mechanics can’t test if something is inevitable or not. You can’t rewind time and start over.
    – user62907
    Sep 3, 2023 at 2:43
  • 2
    How might God reveal himself? Sep 3, 2023 at 2:53

5 Answers 5


In statistics there are varying degrees of randomness and even algorithms for calculating the randomness of given data.

But in philosophy randomness has a different meaning. Here random refers to everything that is "not deliberately decided for a reason".

Every event is caused, but causes never determine their effects with absolute accuracy. Only the probability distribution of the effect is determined by the cause. The actual effect is random within the constraints of the probability distribution. This means a wide variety of outcomes at quantum level and quasi-deterministic behaviour at macro scale.

So, the answer is yes: Probabilistic reality does not follow the principle of sufficient reason. There is no such thing as absolute accuracy.

  • But can't we consider randomness as a cause? That would solve the matter. Incidentally, it would also provide a cause for winning at the lottery: I was just lucky.
    – Olivier5
    Sep 30, 2023 at 9:08
  • Randomness is not a cause. Randomness is the inaccuracy between a cause and its effect. Sep 30, 2023 at 10:07
  • In truth, I agree that there's something wrong with the idea of randomness being seen as a cause, but I can't put my finger on the problem, so I hoped you could help.
    – Olivier5
    Sep 30, 2023 at 14:36
  • You seem to suffer from a common misconception that random means "without a cause". You should understand that in a probabilistic world the cause determines only the probability distribution of the effect. The actual effect occurs randomly within said probability distribution. Radioactive decay of a single atom occurs randomly with 50% probability within a timeframe called half-life. Sep 30, 2023 at 17:21
  • That's a bit condescending and needlessly confrontational. I agree with your general take. The meaning of "random" is not carved in stone, though.
    – Olivier5
    Sep 30, 2023 at 18:11

It's a good question. By cause and effect we mean A happened then B happened because A happened. With quantum mechanics (as we understand it today) the link between A and B is less clear cut because of the probabilistic nature of quantum results. It's worth digging into that nature ...

If A is 'I threw a dice' and B is 'it was a six', then we are simplifying the chain of cause and effect. The dice didn't land with six facing upwards just because I threw it- there were lots of other factors at play, such as its linear and angular momentum when it left my hand, the interaction between the dice and the air, the exact height and position of my hand relative to the surface upon which the dice was dropped etc etc etc. Were we able to know all those factors, we might be able to work out for certain that the dice was going to land as a six, so the probabilistic nature is really a result of our inability to see and follow a more detailed chain of cause and effect. With quantum theory we are in a somewhat different position. We can calculate and assign probabilities to quantum interactions, but as yet nobody has been able to suggest what the underlying chain of events might be that cause one of the outcomes to occur. Crucially, indeed, there are theorems that prove that the probabilities associated with quantum effects cannot be accounted for by some underlying chain of events in the way that the dice roll can be, unless you abandon the idea that instantaneous action at a distance is unphysical.

So, whereas with the dice we know the outcome must be a number between one and six and we could in principle figure our which if we could follow the underlying deterministic chain of events closely, with quantum mechanics we are currently forced to conclude that in principle there cannot be an underlying chain of events. We can say the dice landed on a six because of n factors, but we can't say the same about quantum outcomes- we can't bridge the gap between 'this happened' and 'then that happened' in a way that allows us to identify a chain of cause and effect that makes sense from a common sense perspective.

To put it another way, if I knew all of the starting conditions in sufficient detail, I could figure out that your dice throw would land on a six. With quantum mechanics you cannot, in principle, figure things out with that kind of certainty.

That said, however, at a macroscopic level most quantum indeterminacy disappears, so the principle of sufficient reason holds good in any domain in which you can meaningfully apply classical physics.

  • If I threw a dice, and then the dice still behaved indeterministically before landing on a 6, doesn’t that still imply that the dice landed on a 6 because of a reason? Secondly, even if the laws of physics were completely deterministic, the laws themselves could exist for no reason. So it seems that something being uncaused need not be random and something being random need not be uncaused, no?
    – user62907
    Sep 3, 2023 at 6:36
  • The other trouble I’m having understanding is how events that are indeterministic by themselves suddenly become deterministic in terms of their probabilities. If those quantum events were completely random, wouldn’t it also be impossible to attach a probability? If they aren’t, then what exactly is the reason the probabilities are the way that they are? Or is there none?
    – user62907
    Sep 3, 2023 at 6:38
  • They're not completely random. The way QM works is amazing when you understand it, but quite hard to explain succinctly. Very crudely, every particle has a 'wave function', the shape of which depends on the particle's environment and state. The wave functions form a family, in a particular mathematical sense, so you can always express one wave function in terms of a mix of the others. For example, function A might be expressible as 62% function B, plus 17% function C, plus 3% function D plus 28% function E. When a quantum transition happens... Sep 3, 2023 at 7:06
  • ... the initial wave function of the particle 'jumps' or 'collapses' to become one of the other functions in the wider family of allowed functions, and the probability of it jumping to one or another depends on how much of each function was present in the mix of the initial function. So using the example I gave, there would be a 3% chance of a particle in state described by function A to jump to a state described by function D. I've simplified that a lot, but the gist is right. Sep 3, 2023 at 7:10
  • The 'jumps' happen when a particle interacts with something, such as a measuring device. In traditional QM the jumps are instantaneous. If you study it you will realise that QM is very full of holes conceptually, and in practice is a huge approximation, in spite of which it is capable of being used to make models with ultra accurate predictions of experimental results. Sep 3, 2023 at 7:13

OP: "The Principle of Sufficient Reason is a philosophical principle stipulating that everything must have a reason, cause, or ground."

That is to say, every thing must have a reason. An alpha particle is a thing, produced by decay, but half-life is a span of time, not a thing. Neither is probability a thing in the required sense.

Reinforcing the point with quotes from Heidegger's The Principle of Reason.

[p 44] "Nihil est sine ratione": "Nothing is without reason." Every being has a reason. The subject of the principle of reason is not reason, rather: "Every being"; this is predicated as having a reason. The principle of reason is, according to the ordinary way of understanding it, not a statement about reason, but about beings, insofar as there are beings. ...

[p 49] "Nothing," that is, no being whatsoever "is—without reason."

[p 51] According to the principle of reason, only beings are ever grounded.


Does quantum mechanics rule out the principle of sufficient reason?

Quantum mechanics is based on indeterminism not randomness. Randomness is the unpredictability of events in which each event has an equal probability of occurring. Indeterminism does not require equal probabilities for events but is still unpredictable. This allows for a wave function that contains information as a distribution of the most probable positions. This subtle distinction is why quantum mechanics does not rule out the principle of sufficient reason.

  • can random events have a reason?
    – user67675
    Sep 27, 2023 at 23:27
  • @prof_post When it rains, the drops hit the ground at random positions, the reason for the rain can be explained by someone with expertise in meteorology or atmospheric physics. Sep 27, 2023 at 23:31
  • so there are no true randomness then, i get it
    – user67675
    Sep 27, 2023 at 23:34
  • @prof_post now since yourself claimed you've got it at the reductional bottom QM level, then perhaps could penetrate further on the huge difference between 'error' and 'anything goes' discussed in your recent moral error theory post... Sep 28, 2023 at 5:23

No answer deals with the key element of the question: causality.

Causality is the rational notion that tells that actions trigger reactions (A->R).

It is within the context of causality that the POSR requires an A to exist for R to be possible. So, the POSR is the same as causality, except that it is, in final terms, expressed in modus tollens form (~R -> ~A).

Notice I've highlighted the concept rational notion. Causality exists in our mind, not as a physical fact. When we see that a ball moves after it gets hit by another, in fact, all the things in the world are not acting and reacting, but every atom is interacting with every other atom. Causality is our rational interpretation of such fact.

Quantum mechanics does not fit with out notion of causality, so you can't put both behaviors in the same bag. QM has no relationship with the POSR (which is different to the fact that the QM formalism is based on causality, yes, but it does not rule out the POSR).

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