I am having trouble deciding between the difference and whether this difference really is real.

For example, quantum mechanics has indeterministic effects. The time at which an atom decays has a certain probability or an electron’s position after it goes through two slits. Presumably, this means that both of these things occur for no reason.

In other threads on this matter, I read the argument that if something occurs indeterministically, it doesn’t imply that it occurs for no reason. But I’m having trouble wrapping my head around this. If there is nothing in the world that can help me figure out exactly when an atom decays, and can only have probabilities, in what way could there be any reason (even if ”insufficient”) for it to decay at time t?

Even if there were only two possible times with a probability of 50% each, how can there be a “partial” reason for it to decay at time t1 instead of time t2? Furthermore, even if there was some god that somehow determined this, then that determination would be the reason. This would then ultimately function as a “non local” hidden variable theory.

Hence, in what way can something be not deterministic but also occur for some (insufficient) reason? I’m failing to see how they are not one and the same.

Lastly, I wanted to address the notion of logical contradictions in this concept. Are there any logical contradictions in the idea of singular atoms decaying at time t for no reason but a group of atoms decaying at an average time t for a reason? How does one show that this is logically possible in the first place?

  • X is a reason for Y when it is a necessary condition for Y to happen. It is a sufficient reason when it is also a sufficient condition. An atom has to be an atom to decay, and a particular atom at that to calculate probabilities, half-life, etc., so those are reasons for it to decay. They are insufficient because even when all the necessary conditions are present it may still not decay. But when something known to decay does decay, and in agreement with as much as necessary conditions predict, we are not surprised because even insufficient reasons explain the occurrence enough.
    – Conifold
    Nov 1, 2023 at 12:43
  • @Conifold Me existing is a necessary condition for me to wake up tomorrow. Yet no one would consider me existing as a reason (even partial) for that. Rather, one could just say it makes no sense to talk about me waking up without me existing, not that my existence is a reason for it. Perhaps this is just a semantic issue but it seems that this terminology uses the word “reason” to mean something that it doesn’t or ever did in historical parlance.
    – user62907
    Nov 1, 2023 at 18:41
  • Sure it did. Throwing a ball into a window is traditionally considered a reason for the window getting shattered. Yet the reason is not sufficient, a gust of wind could have blown it off course. In fact, you never met a sufficient reason in your life, if you think about it, it is always a projected theoretical abstraction. As for common parlance, some necessary conditions are pragmatically singled out as named "reasons" against a background taken for granted, but that is ontologically irrelevant. Your existence is as much a reason for what you do as what is named as such colloquially.
    – Conifold
    Nov 1, 2023 at 19:10
  • @Conifold I just don’t agree with a reason being a special kind of necessary condition as you stated then. Throwing a ball into a window is not a necessary condition for the window to break. However, it functions as a reason for the window breaking in common parlance if one has evidence for that throw to have broken the window. The ball’s existence, the window’s existence, etc. however are both necessary conditions for the window to break. And yet, neither of them would be considered reasons. And neither of them would be “just as much” of a reason as the throw for the window to break.
    – user62907
    Nov 1, 2023 at 20:02
  • 1
    @ScottRowe Not being able to predict a result does not imply that it is impossible in principle. We don’t actually observe a lack of reason for each dice throw. We just observe the dice throws.
    – user62907
    Nov 2, 2023 at 0:25

3 Answers 3


Generally a reason is sufficient if by itself it can explain the observed effect. A sufficient reason for my yard flooding is that I was too lazy to clear the drains and there was very heavy rain. Neither my laziness nor the heavy rain is a sufficient reason although both can be considered a reason. You can consider that in a different way if you wish. For example, you might say that given I had let the drains fall into a deplorable state, the heavy rain was a sufficient reason, or you might say that given the likelihood of heavy rain, my poor maintenance of the drains was a sufficient reason, but that just goes to show that the difference between a sufficient reason and a reason is largely a matter of wordplay.

On a related point, suppose atoms did decay at random with no trigger event, you could still maintain that quantum events being spontaneous is the reason.

  • To your last point, it seems that would only qualify if a reason can be defined as just a rephrasing of the event. Saying that a quantum event is spontaneous is just saying that it is random with no trigger event. This equates to saying that the reason for A is A.
    – user62907
    Nov 1, 2023 at 9:44
  • 2
    Agreed, sort of. I think my point is that the word reason can either mean 1) a specific trigger event or 2) an explanation or categorisation. So if something happened as if by magic, then a statement such as 'this happened by magic' or 'this happened for some unknown reason' might still just about qualify as a reason type 2). Nov 1, 2023 at 10:25
  • I think homing in on trigger events is the way to go. In the two slits experiment, the electron is found somewhere on the screen. Before that, it had a wave function that was more spread out. There must have been some trigger event that caused the localisation. Nov 1, 2023 at 10:28
  • A famous guy said, "The two-slit experiment is the only mystery."
    – Scott Rowe
    Nov 1, 2023 at 11:11

There could be many kinds of "reason". But I suppose that in physics, a sufficient reason would be a sufficient cause for a given effect, ie it presupposes causality, and a sufficient cause would then be one that "is enough" to explain a given observation, fact or event.

Given that radioactive decay and other quantum phenomena are modeled as probabilistic, the current models do not offer a sufficient cause for an atom to decay at time t.

But they provide a necessary cause, hypothetical at least, which is based on the observations that some isotopes are more stable than others; they decay at different speed (ie they have halflives, a very useful regularity). Some isotopes appear stable. So the necessary cause must lie in some instability withing certain atomic nuclei.

So unstable isotopes decay faster than stable ones. And this provides a necessary but not sufficient reason for any singular radioactive decay at time t. The nucleus in question could very well have decayed much sooner, or much later.

In philosophical parlance, the timing of the event is contingent.

In New Yorkese, shit happens.

  • Right, we can't see inside. If you put marbles in a bowl and swirl it around, one will probably fly out eventually. Saying exactly when and why is impossible, and doesn't really get us very far. Just work around it.
    – Scott Rowe
    Nov 1, 2023 at 11:08
  • What do you mean by “necessary cause”? Necessary conditions have never meant causes historically. It is a necessary condition for you to exist before you die. But no one would say that your existence is a cause or reason for your death in any capacity. Rather, your existence is a notion without which it makes no sense to talk about death. A necessary precursor for X isn’t a cause for X. It just says that X cannot happen without Y, not that X is caused by Y. The old correlation != causation trope.
    – user62907
    Nov 1, 2023 at 18:45
  • @thinkingman Kant spends eons talking about causes as necessary conditions, so where are you getting your reading of history from??? Nov 1, 2023 at 23:34
  • Causality is different from necessary/sufficient in that the former is a metaphysical relation between events and the latter terms are about logical conditions.
    – user62907
    Nov 2, 2023 at 0:23
  • @thinkingman "But no one would say that your existence is a cause or reason for your death in any capacity." Those with suicidal ideation would strongly disagree.
    – Notso
    Nov 2, 2023 at 3:25

Great question. Determinism seems to break down at the quantum level. I find it surprising that this does not seem to undermine determinism altogether in the eyes of some philosophers. If there is a difference between a reason and a sufficient reason but it is indiscernible then they are logically identical, according to Leibniz's principle of indiscernibles. Using Occam's Razor, I am inclined to agree that the are the same.

  • Famous guy said that things can only be fully explained in terms of other things at a different level. Bricks are red, but molecules are not. So determinism can only properly be explained via non-deterministic activity. Otherwise it is question-begging? Cheers
    – Scott Rowe
    Nov 1, 2023 at 11:14

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