The logic of science-8: The power of universal claims in science

(For other posts in this series, see here.)

In the previous post in this series, I argued that in the case of an existence claim, the burden of proof is upon the person making the assertion. In the absence of a preponderance of evidence in its favor, the claim can be dismissed. As has often been said, “What can be asserted without proof can be dismissed without proof”. The basis for this stance is the practical one that proving the non-existence of an entity (except in very limited circumstances) is impossible. Hence if we do NOT have a preponderance of evidence in favor of the existence of an entity, we conclude that it is not there.

In the case of a universal claim, however, the situation is reversed and the default position is that the claim is assumed to be true unless evidence is provided that refutes it. So in this case, the burden of proof is on the person disputing the assertion, again for eminently practical reasons.

As an example, the universal claim that all electrons have identical masses and charges can never be proven to be true with just supporting evidence because we cannot measure the properties of every electron in the universe. But once a few of them have been shown to have the same mass and charge, the universal claim that all of them do is presumed to be true unless someone comes up with evidence that disputes it. This is why the proposition “All electrons have the same mass and charge and behave identically in interactions with other particles” is believed to be a true proposition. In this case, absence of evidence (against the universal proposition) can be taken as evidence of absence that such evidence exists at all.

In science, negative evidence can be powerful in the same way that it can be in the legal setting, as in the famous Sherlock Holmes story of the inferences that can be drawn from the dog that did not bark in the night. Since there is a belief that dogs always bark when unexpected events occur in the night, we can infer from a silent dog that nothing untoward happened. In science, we believe that natural laws are invariably followed without exception. For example, the strongly held scientific belief that there exist only two kinds of electric charge is based entirely on this argument, because there has been no evidence produced that we need a third kind of electric charge. Similarly, any universal claim about the properties of an entity whose existence has already been established are taken to be true unless evidence is provided that contradicts the claim.

The laws of science are (as far as I am aware) always phrased as universal claims. There are a number of such laws such as energy, momentum, and angular momentum conservation, baryon number conservation, and CPT conservation (where C stands for ‘charge conjugation’, P stands for ‘parity’, and T for ‘time reversal) all of which are believed to be true purely because no violations have been observed. Anyone who challenges the validity of those laws has the burden of proof to provide evidence of such violation. This approach is so routine in science that no one even bothers to state it explicitly

The contradiction of a universal claim is done by means of an existence claim. For example, it used to be considered that something called CP was also conserved in every reaction. Why did we believe this universal claim? Because no reaction violating it had ever been seen. But some scientists suspected that it might be violated under certain conditions. Postulating such a reaction constituted a new existence claim. This was not initially accepted since no one had seen a violation of CP. But then one rare reaction was detected that did violate CP and this was confirmed in subsequent experiments. It was only then that the universal claim that CP was never violated was accepted as not being true, because some researchers produced evidence in support of their existence claim of such violations. Now, without further evidence, we are justified in believing the universal claim that this same reaction will violate CP every time it happens, until someone finds evidence for the claim that on occasion it does not.

So in science this interplay of existence and universal claims, and the different ways they are established, goes on all the time and forms an integral part of the way that scientific knowledge is constructed.

‘God exists’ is an existence claim and the burden of proof lies with those who assert it. In the absence of such evidence, the scientific conclusion is that god does not exist. Similarly, ‘god does not exist’ is a universal claim and the burden of proof lies with those who deny it and they must again provide evidence that god exists. Since they have not produced any such evidence, the scientific conclusion is that ‘god does not exist’ is a true statement.

It necessarily follows from the above discussion that in science the word ‘true’ is used provisionally and not absolutely. In the case of an existence claim, ‘true’ is taken to mean that it is supported by a preponderance of evidence. In the case of universal claims, ‘true’ is used as an abbreviation for ‘not yet shown to be contradicted by evidence’. It is always within the realm of possibility that someone might come along with data that suggests that there exists a particle that seems to behave identically as the electron but has (say) a different mass. In fact, that has actually happened. The scientific community responded with further experimentation that confirmed the existence of this new particle, now called the muon, and it is now considered a true proposition that muons exist and all have the same mass and charge and behave just like electrons except that their mass differs from that of electrons.

Maybe one day there will be a preponderance of evidence for the existence of god. But until such time, a perfectly valid scientific conclusion is that god does not exist.

Next: Proofs as used in science


  1. Manik says

    The people who come up with these arguments are not “true” believers. The human mind cannot begin to understand God. If we could, by definition ‘he'(for the want of a better word)is not god. Unfortunately most “believers” want to justify their belief in God. Can a blind man describe an Elephant? God is the creator, the almighty, the most merciful, omniscient, omnipresent. That’s all you need to believe. I don’t understand Space-Time, Anti Matter, String Theory, more than 3 dimensions etc.,etc. How can I ever hope to understand god’s nature? The simplest answer is generally the best. For a person of Faith no Explanation is necessary, for a person without no Explanation is Possible. All you require is a childlike faith. I am equally surprised that many Atheists do not seem to grasp this themselves. All this to-ing and fro-ing is an exercize in futility. Keep it simple stupid! Both parties should accept this simple premise and move on.

  2. Robert Allen says

    What about the black swan fallacy: All the swans I see are white, therefore “all swans are white” until I find a black swan. I don’t think you can make universal claims unless there is a good reason *why* swans are white.

    Another problem I see with your electron example is that charge and mass are part of the *definition* of an electron. So, if you did find something that had a different charge or mass, you’d simply categorize it as something else. So I don’t see that as a universal claim so much as just a definition. “If we find a particle with such and such properties, we’ll call it an electron. Otherwise, call it something else.” It isn’t really claiming anything universal, is it?

    The CP conservation example is better, however, wouldn’t we still need an explanation of why CP should be conserved, rather than just saying “well, we’ve observed it to be conserved so far…”?

  3. says


    I don’t think that it is necessary to have a theoretical reason for an empirical universal claim. If all the swans that you have observed so far are white, then making the claim that all swans are white seems reasonable. Such a claim might spur an investigation into why this is so, but is not dependent on an answer being obtained. The same would be true for CP violation. Many scientific laws started out (and some still are) as purely empirical generalizations.

    As for the electron, the question of what is part of a definition and what is not is tricky. One could define an electron as a particle with a particular mass and charge but there are alternative definitions. For example, the OED defines it as “A subatomic particle which has a negative electric charge equal in magnitude to the positive charge of the proton, is a constituent of all atoms, and is the principal carrier of electric current in solids”, without any statement of mass. The OED gives a more complete list of the shifting definitions of an electron through time.

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