A reader sent me a link to a comment on one of my least favorite major creationist websites, Uncommon Descent (No link, I refuse to link to UD). It’s dumb enough that it really deserves a good mocking.

Barry Arrington, June 10, 2016 at 2:45 pm

daveS:

“That 2 + 3 = 5 is true by definition can be verified in a purely mechanical, absolutely certain way.”This may be counter intuitive to you dave, but your statement is false. There is no way to verify that statement. It is either accepted as self-evidently true, or not. Think about it. What more basic steps of reasoning would you employ to verify the equation? That’s right; there are none. You can say the same thing in different ways such as || + ||| = ||||| or “a set with a cardinality of two added to a set with cardinality of three results in a set with a cardinality of five.” But they all amount to the same statement.

That is another feature of a self-evident truth. It does not depend upon (indeed cannot be) “verified” (as you say) by a process of “precept upon precept” reasoning. As WJM has been trying to tell you, a self-evident truth is, by definition, a truth that is accepted because rejection would be upon pain of patent absurdity.

2+3=5 cannot be verified. It is accepted as self-evidently true because any denial would come at the price of affirming an absurdity.

It’s absolutely possible to *verify* the statement “2 + 3 = 5”. It’s also absolutely possible to *prove* that statement. In fact, both of those are more than possible: they’re downright *easy*, provided you accept the standard definitions of arithmetic. And frankly, only a total idiot who has absolutely no concept of what verification or proof *mean* would ever claim otherwise.

We’ll start with verification. What does that mean?

*Verification* is the process of testing a hypothesis to determine if it correctly predicts the outcome. Here’s how you *verify* that 2+3=5:

- Get two pennies, and put them in a pile.
- Get three pennies, and put them in a pile.
- Put the pile of 2 pennies on top of the pile of 3 pennies.
- Count the resulting pile of pennies.
- If there are 5 pennies, then you have
*verified*that 2+3=5.

Verification isn’t perfect. It’s the result of a single test that confirms what you expect. But verification is repeatable: you can repeat that experiment as many times as you want, and you’ll always get the same result: the resulting pile will always have 5 pennies.

*Proof* is something different. Proof is a process of using a formal system to demonstrate that within that formal system, a given statement necessarily follows from a set of premises. If the formal system has a valid model, and you accept the premises, then the proof shows that the conclusion must be true.

In formal terms, a proof operates within a formal system called a *logic*. The logic consists of:

- A collection of rules (called
*syntax rules*or*formation rules*) that define how to construct a valid statement are in the logical language. - A collection of rules (called
*inference rules*) that define how to use true statements to determine other true statements. - A collection of foundational true statements called
*axioms*.

)

Note that “validity”, as mentioned in the syntax rules, is a very different thing from “truth”. Validity means that the statement has the correct structural form. A statement can be valid, and yet be completely meaningless. “The moon is made of green cheese” is a valid sentence, which can easily be rendered in valid logical form, but it’s not true. The classic example of a meaningless statement is “Colorless green ideas sleep furiously”, which is syntactically valid, but utterly meaningless.

Most of the time, when we’re talking about logic and proofs, we’re using a system of logic called first order predicate logic, and a foundational system of axioms called ZFC set theory. Built on those, we define numbers using a collection of definitions called Peano arithmetic.

In Peano arithmetic, we define the natural numbers (that is, the set of non-negative integers) by defining 0 (the *cardinality* of the empty set), and then defining the other natural numbers using the *successor function*. In this system, the number zero can be written as ; one is *(the successor of )*; two is the successor of 1: . And so on.

Using Peano arithmetic, addition is defined recursively:

- For any number , .
- For any number numbers x and y: .

So, using peano arithmetic, here’s how we can prove that :

- In Peano arithemetic form, means .
- From rule 2 of addition, we can infer that is the same as . (In numerical syntax, 2+3 is the same as 1+4.)
- Using rule 2 of addition again, we can infer that (1+4=0+5); and so, by transitivity, that 2+3=0+5.
- Using rule 1 of addition, we can then infer that ; and so, by transitivity, 2+3=5.

You can get around this by pointing out that it’s certainly not a proof from first principles. But I’d argue that if you’re talking about the statement “2+3=5” in the terms of the quoted discussion, that you’re clearly already living in the world of FOPL with some axioms that support peano arithmetic: if you weren’t, then the statement “2+3=5” wouldn’t have any meaning at all. For you to be able to argue that it’s true but unprovable, you must be living in a world in which arithmetic works, and that means that the statement is both verifiable and provable.

If you want to play games and argue about axioms, then I’ll point at the Principia Mathematica. The Principia was an ultimately misguided effort to put mathematics on a perfect, sound foundation. It started with a minimal form of predicate logic and a tiny set of inarguably true axioms, and attempted to derive all of mathematics from nothing but those absolute, unquestionable first principles. It took them a ton of work, but using that foundation, you can derive all of number theory – and that’s what they did. It took them 378 pages of dense logic, but they ultimately build a rock-solid model of the natural numbers, and used that to demonstrate the validity of Peano arithmetic, and then in turn used that to prove, once and for all, that 1+1=2. Using the same proof technique, you can show from first principles, that 2+3=5.

But in a world in which we don’t play semantic games, and we accept the basic principle of Peano arithmetic as a given, it’s a simple proof. It’s a simple proof that can be found in almost any textbook on foundational mathematics or logic. But note how Arrington responds to it: by playing word-games, rephrasing the question in a couple of different ways to show off how much he knows, while completely avoiding the point.

What does it take to conclude that you can’t verify or prove something like 2+3=5? Profound, utter ignorance. Anyone who’s spent any time learning math should know better.

But over at Uncommon Descent? They don’t think they need to actually learn stuff. They don’t need to refer to ungodly things like textbook. They’ve got their God, and that’s all they need to know.

To be clear, I’m not anti-religion. I’m a religious Jew. Uncommon Descent and their rubbish don’t annoy me because I have a grudge against theism. I have a grudge against ignorance. And UD is a huge promoter of arrogant, dishonest ignorance.