It seems that with certain assumptions about reality, everything we can do is equally rational. Max Tegmark's theory of the multiverse, often referred to as the "mathematical universe hypothesis", suggests that all physical reality we observe can be fundamentally described by mathematical structures. In his book, Our Mathematical Universe: My Search for the Finite Nature of Reality, Tagmark explores this concept in detail. On page 380 he says:

"But if there are parallel universes where all physically possible futures play out, why should we care about our own Universe? If all outcomes will happen, why should we care about what choices we make? Indeed, why should we lift a finger or care about anything at all if the Level IV multiverse exists and even change itself is an illusion? We face a choice between two rational alternatives:

  1. We care about at least something, and therefore go ahead and live life, making logical decisions reflecting the things we care about.

  2. We care about nothing, and therefore do nothing at all or act completely randomly.

Both you and I have already made our choices, selecting option 1."

But do these assumptions that have been mentioned, that there are no real changes, or that there are parallel universes where all the physically possible options for the future are at play, really create problems for rationality like this? (That everything we can do is equally rational)? Are there any other plausible assumptions that challenge rationality in this way? Although Tagmark later says that option 1 seems like a smart choice to him, by the phrase "two rational alternatives" he seems to imply that his theory establishes physical determinism and something like block determinism (fatalism), which have these consequences for rationality (that “future is inevitable” due to this, that everything is “pointless”).

Determinist and fatalist arguments have the same conclusion, that the future is somehow fixed and not within our control, but the former do so from causal or nomological considerations while the later do not.

Source. According to what I found on the Internet,

“In fact, it is not clear at all whether block determinism rules out libertarian free will… For the classical compatibilist, after all, free will does not require the ability to do otherwise; it merely requires the ability to do what one wants. On such a reading, then, free will is perfectly consistent with a causally fixed, unique, and fully determinate future”.

(Source, p.16). Nevertheless, The Stanford Encyclopedia of Philosophy says (about block determinism)

"It is easy to see that, by adjusting the speeds of Alice and Carol, any event to the future of O can be shown to be fixed or real or inevitable”.

Source To sum up, while block determinism may not correspond to a certain concept of free will, it implies that all events are fixed or real or inevitable. This raises the question, as with physical determinism, whether everything we can do will be equally “rational alternatives”, and whether there are any plausible assumptions other than block and physical determinism that would lead us to such a conclusion.

  • This is an intuition that pops up actually pretty frequently in philosophy. I don't think it's correct, but I understand why it seems intuitive to people. If the future is determined, then a rational person is as determined to think rational thoughts as an irrational person is to think irrational thoughts, and thus both of them are essentially ... the same?
    – TKoL
    Jan 8 at 11:27

4 Answers 4


To be rational consists in acting according to what our senses say is the case, which also requires us to act according to whatever logical implications follow from that. This has nothing to do with what the universe might really be.

To be rational doesn't mean that we are right. We may be 100% rational and still 100% wrong. However, it seems that, overall, we are less likely to be wrong if we are rational; and most people are, because if they weren't, humanity itself just wouldn't survive. It wouldn't even exist to begin with.

One aspect of rationality is that we see the future as open, full of alternative possibilities. Yet, this only means that we don't know what is going to happen next, and so we edge our bets according to what we believe is likely to happen. We take a life insurance not because we know that we are going to die, but because we believe that we are going to die and we don't know when. Reasonable life is then to make contingency plans for the more probable. Those who don't tend on average to fare less well.

So, to behave rationally, someone who would seriously believe that they are going to personally experience all possible alternative futures would be thereby motivated to take at the same time all contingency measures necessary to care for all of these alternative futures, and would therefore be subject to a level of stress so enormous that they would die on the spot.

This is presumably not what the question is about.

Rather, we are probably asked to consider someone who take seriously the possibility that they are equally likely to experience any one of the said alternative futures. That is to say, someone who hasn't a clue how the real world is. A total agnostic. How to be rational in such a situation? This is a very unlikely scenario, for to be able even to envisage alternative future scenarios, you need to have some experience of how it went so far, and so you need an experience of the real world, which presumably would leave you with some beliefs about what futures are more probable.

Still, someone with an experience of the world sufficient to consider alternative futures but somehow unable to attribute more probability to any one scenario would not be in the same position as the case already discussed, for they would see all scenarios as having exactly the same probability, possibly close to zero. They would be in the same position as Buridan's ass, unable to decide what to do between equally probable futures, because everything they could think of doing would be just as justified as anything else. So, like Buridan's ass, they would die from doing nothing.

And this explains why there is nobody like that. They don't reproduce.


Most of us consider rationality to be an approach we can take to decision-making in the context of real choice. Per determinism, we HAVE no real choice, so this idea of "freedom" is an illusion.

Compatibilists have redefined freedom, and willing, and rational decision-making, from the concept that the rest of us use, in order to find a way to reconcile between their belief in a determinist universe, and their belief in rationalism. Most people look at a compatibilist cross-eyed, when compatibilists say they make free choices, but are also determined. But enough philosophers have convinced themselves that this apparent contradiction makes sense, that it is the majority position in philosophy today.

There are problems with this view, which your reference to Tegmark and the multiverse hint at, but yor question does not highlight.

The primary problem for compatibilists is that our universe is NOT deterministic. Quantum Mechanics is not, it is probabilistic. The efforts to match its data using deterministic local hidden variables have failed, and using non-local hidden variables are a different theory, called Bohmian Mechanics, and it is trending toward refutation too. https://settheory.net/Bohm

"Determinists" who recognize the victory of QM have in many cases adopted the Everett or Tegmark multiverse interpretation, but these two fail to actually satisfy the criteria of determinims. Even a God or knowledge demon cannot know the state of the universe at any time (per Heisenberg uncertainty), nor know what future one will experience after a decision (because an infinite number of them will happen, and a version of "you" will have made each choice, and experience each outcome). This isn't actually determinism -- there remains a probabalistic term as to which of these futures "you" will experience.

These multiverse models, if thought through, have a catastrophic effect on reasoning. IF the I of this moment, given my history and character, could complete this post, then go on to a productive day, OR blow off all my responsibilities and waste the day playing video games, OR walk next door and kill the neighbors, AND I will actually do ALL of these things per multiverse theory, AND in each case there are an infinity of different consequences that will befall each of these three versions of me -- there can be no rational decision-making. Probabilities of outcomes do not apply when all outcomes will happen. Developing character and good habits of thought do not matter when all outcomes will happen, etc. And one cannot even try to apply a pseudo-probability to the frequency of lives that will experience each of the above -- all are actually infinite in number, to the same order, so such probabalizing to minimize "bad" futures cannot work per math theory.

So -- determinism contradicts the common assumptions we have about our freedom and decision-making (key to reasoning), both need to be redefined by compatibilists. AND determinism is refuted by QM. AND Tegmark's multiverse takes away even compatibilist choice. The coherence of rationality is hard to maintain in the face of these problems, without adopting a libertarian free will causal logic. Therefore it is more an issue that nothing we do may be rational, rather than all being rational.


Some people intuitively feel like determinism creates a situation where nothing matters, all choices are 'interchangable' or equally 'rational', because 'rationality' means nothing in a deterministic world.

I personally don't agree, because I see 'rationality' as an algorithmic tool, and that algorithm doesn't rely on true indeterminism to be effective. I don't think indeterminism does, or even can, add anything of value to human existence (I accept the very real possibility that we are in fact in a somewhat-indeterministic universe). In the same way that one knife may be sharper than another one, and thus better at cutting - even in a deterministic world - one mind may be more rational than another one, and therefore better at... whatever rationality is good for. Decreasing our chances of being terribly wrong? I don't know.


No kind of determinism is a plausible assumption in the context of human decision-making.

There is nothing "rational" and no concept of "alternative" in determinism.

  • If so, then why is the fact that determinism implies an absence of rationality not a sufficient reason to reject it? Why don't all scholars deny it? Jan 7 at 13:56
  • @AromaticInternal624 Determinism is not something you could accept or deny. Determinism is only a theoretical idea of an imaginary system, not a claim or a theory about reality. Jan 7 at 14:34
  • There are different definitions of determinism Determinism is... the theory that all events in the universe are completely caused by prior events… [Source] (researchgate.net/publication/352661459_Determinism) the claim that every event in the universe is determined [Source] (pressbooks.online.ucf.edu/introductiontophilosophy/chapter/…) You see, the argument from reason has followers and opponents. So could you please refer to the sources where it is written that if determinism is true, there is no rationality? Jan 7 at 17:10
  • @AromaticInternal624 Determinism is neither true nor false. Determinism is not a truth claim or a theory. Your sources are wrong about that. The former describes conditions known to be different from reality. The latter one is a tautology of a sort, it doesn't tell us what "determined" means. Determined by what? Jan 7 at 17:23

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