(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