# The nature of proof

The word “proof” carries a lot of meanings in English, as does the associated verb form “prove”. The meaning that most people generally think of when they see the word “proof” is something like:

1. an argument or chain of logic that establishes something as a fact, beyond any reasonable doubt

This is the sense of a mathematical or logical proof. Using only very basic and agreed axioms of geometry, you can prove Pythagoras’s theorem: that the square of the length of the hypotenuse of a right angle triangle equals the sum of the squares of the other two sides. This is a mathematical fact that anybody who follows the logic of the proof cannot sensibly dispute.

Another meaning of the word “proof” is:

2. a test or trial, for example of a proposition

This sense of the word is seen in the proverb “the proof of the pudding is in the eating”, which means that you have to test the pudding by eating it, or figuratively that the only way to establish that something is correct or does what you want is to test it.

Similarly, two meanings of the verb “prove” are:

1. to provide an argument or evidence that establishes something as the truth

2. to put to the test, or to put to trial

Although for each word the first meaning is more commonly understood, the words trace their etymology back to the Latin probare, meaning to probe or to test, so the second meaning is historically closer to the origin.

Science proves things by testing them, not by establishing them as incontrovertible fact. Indeed, if something cannot be put to the test and—in principle—be disproven by contradictory evidence, then it’s not really in the realm of science.

The “100 Proofs” that the Earth is a Globe on this site are not “100 arguments or bits of evidence, each one of which by itself establishes as a fact beyond any possible counter-argument that the Earth is spherical”. That’s not how science works. For each of the 100 Proofs that I’ll be publishing, it is certainly possible to construct a counter-argument. For example:

1.Photos of the Earth from space do not establish that the Earth is round, because NASA could have faked them.”

2.Eratosthenes’ measurement does not establish that the Earth is round, because maybe light doesn’t travel in straight lines, or maybe the sun is only a few thousand kilometres away.”

What each of the “100 Proofs” are is a test of the proposition that the Earth is a globe. They are also tests of the proposition that the Earth is flat. The way science works is that we observe something in the universe, and we construct a model which we think explains it. When we first observe the Earth from our position on it, we might construct two different models: (a) The Earth is a globe. (b) The Earth is flat. Which model is more likely to be better represent reality? For that we use the scientific method – we test the models by making predictions and then doing experiments to see if they pass the test.

1a. If the Earth is a globe, what shape should we expect to see from space? A circle, with half of the Earth visible (the other half being on the back side), partly light by the sun as a ball would be. Do we see that? Yes.

1b. If the Earth is flat, what shape should we expect to see from space? A circle or an ellipse, with all of the Earth visible. Do we see that? No.

2a. If the Earth is a globe (and the sun is very far away), would the shadows cast by vertical sticks at places further north or south be the same? No. Is that what we observe when we do the experiment? Yes.

2b. If the Earth is flat (and the sun is very far away), would the shadows cast by vertical sticks at places further north or south be the same? Yes. Is that what we observe when we do the experiment? No.

Given these pieces of evidence, we can conclude that the globe model is consistent with our observations, but that the flat Earth model is either incorrect, or in need of some sort of revision. At some point, revising the flat Earth model to successfully explain all of the tests—or proofs—we can provide becomes untenable, and we reject it. If the globe model passes all of the tests, then we can conclude that it is a good and useful model of reality.

It might still not be the truth. If any future observation or experiment contradicts the globe model, then as scientists we must either reject that model, revise it, or come up with a better model. This is the way the scientific method works. It doesn’t prove things by establishing them as true. It proves them by testing them. That’s what we’re doing on this site.