Looking at Putnam's twin earth thought experiment, his argument for semantic externalism is based on possibility of their being a twin earth, where everything is the same except for water being made of substance XYZ, instead of H2O.

This is a conceivability based argument, in the sense that it is enough for XYZ to be conceivable - or logically possible - even if it doesn't exist in reality, for Putnam's argument to hold.

But what if substance XYZ isn't even conceivable?

Maybe, if we work up from the laws of particle physics and then quantum mechanics, the only substance that can ever have all the properties of water is H2O, and any other set of atoms and molecules behaving like water is a logical contradiction?

If this is true, then the fact that we can conceive of XYZ is more of a fallacy in our judgement (due to incomplete knowledge about physics) and is closer to a logical contradiction than a mere physical improbability.

Would this also mean that water == H2O (a necessary truth, not a contingent one), and all of the properties of water (transparent, non-reactive liquid, etc...) are necessary truths, not contingent ones?

3 Answers 3


I think we have to back off from the question and see what we mean by 'water'. If we are questioning the structural similarity of water to the substance we know scientifically, what was it before that?

Outside of a given scientific paradigm, water is something with three states that is usually encountered as a liquid, that in that state conducts heat well, and has a certain convenient surface tension behaviors that make it wick into cloth, that expands when it solidifies... In short, it is a collection of behaviors.

But behaviors of a substance are perceptions. It is very hard to imagine that any collection of accidents like perception can prove to have an absolute logical necessity. So it seems down that path your answer is going to be no.

However, these perceptions are mediated by the interface between body and mind. So there is a huge place in the system for psychology. To some other being, constituted quite differently physiologically, the relevant aspects of water for us, might all be met by something else. After all, what is transparent depends upon your eyes, and what flowing means depends on your sense of time, and...

We can take half a step in this direction by looking at something like Robert Forward's 'Camelot at 30 K', where he constructs a society that uses hydrogen flouride the same way we use water, since the major difference between the two substances is that the latter has a very, very low boiling point. He extrapolates that things are slower, so they have to be smaller in order to have the same sort of rhythm of life and otherwise finds reasonable ways to make their experience of H2F be our experience of H2O.

You can imagine continuing down this path of mapping the experiences of more and more exotic beasts onto human experience to consider stranger and stranger substances to be the equivalent of water, manipulating aspects of those variant creature's natures so that their experience of, say, boiling tar, would be our experience of water.

Eventually you could reach the point where 'water' was not two of one thing and one of another, but a compound of three different elements.

We can't get there by working up from more detailed layers of science, because we got those layers by observing our macroscopic universe, and delving it. So when this beast explored its own chemistry and physics, it would not agree with ours.

That would mean that the laws of nature are not only distant from necessary truths, but can take variant forms depending upon what sort of beast derives them.

  • Never thought the possibility that Oscar was not human at all. Thanks. Commented Jan 16, 2016 at 0:27

Concerning your title question

Has anyone ever argued that the laws of Nature and physics are necessary truths?

the answer is yes: Kant, Immanuel: Metaphysical Foundations of Natural Science (1786). E.g., see the propositions in the third part Mechanics of this text.

But contemporary scientists, notably in the tradition emphasized by Karl Popper, refute his claim.

Note: I do not understand why you consider "water = H20" not just a definition. And definitions are neither true nor false.

  • You will note that I used "==" not "=", emphasizing that water is identical to H2O, not just equal to it. This wouldn't be just a matter of definition, since water is a macrscopic substance and H2O is a molecule. Commented Jan 15, 2016 at 20:47
  • @Alexander Is there a difference between identical to and equal to?
    – Jo Wehler
    Commented Jan 15, 2016 at 21:26
  • @AlexanderSKing Water is at least three macroscopic substances. And we almost never encounter any of them in a pure enough state to say that exactly what is there is only H2O molecules. So I am having the opposite problem from Jo. There are so many interpretations of the correspondence that are just not true that it is ambiguous what it really means for the two to be equal, much less identical.
    – user9166
    Commented Jan 15, 2016 at 21:38
  • @JoWehler Yes there is. "Barack Obama" = "President of the U.S" is true but "Barack Obama" == "President of the U.S" isn't, this illustrates the difference. Commented Jan 15, 2016 at 21:38
  • @AlexanderSKing Neither of those is true either. Only "'Barack Obama' in 'Presidents of the U.S.'" makes any sense. The rest is all language being strange to save us time. "President of the U.S." is a function of time, among other things.
    – user9166
    Commented Jan 15, 2016 at 21:41

Putnam uses this reasoning the other way:"we can perfectly well imagine having experiences that would convince us ...that water isn’t H20. In that sense, it is conceivable that water isn’t H20. It is conceivable but it isn’t logically possible! Conceivability is no proof of logical possibility". Hence "water is H20" is what Kripke calls a necessary a posteriori. Reid and Kneale say in agreement with your sentiment that if something is "true for all you know” then it will be conceivable whether it is possible or not. Yablo gives an interesting review in Is Conceivability a Guide to Possibility?

One should note that there are different flavors of possibility (logical, metaphysical, physical, etc.) with correspondingly different notions of conceivability since we are imposing different restrictions on the set of premises that is supposed to be thinkable without contradiction. Putnam also does not need (and can not allow) XYZ to have "all properties of water", only enough to fool Oscar's none too sharp discerning powers. Even if the laws of physics do forbid a substance with all Oscar relevant properties of water, other than H20, it may still be logically or metaphysically possible (however that is defined). That would suffice for Putnam's thought experiment. But I suspect that Putnam does not need even that. His point is about meaning, as long as we can understand twin Earth and XYZ they do not need to be possible. An analogy: we can understand what an odd perfect number is, but such a number may well be logically impossible. Meinong was willing to understand even round squares.

As for the title question, the position that everything that did and will happen must happen, i.e. is logically necessary, is ancient. It is referred to as logical fatalism, Stoics held it among others. It was originally supported by an argument that seems silly today, that whatever is logically true must be true always and forever, and if something happening the way it does is true when it happens, then it is a logical tautology. In modern times Spinoza argued that temporal succession of events is simply a reflection of the logical order of Substance to limited minds it individuates. Kant took a more moderate position by asserting that Newtonian laws were necessary truths, but only in the realm of appearances, not things in themselves.

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