The Halting Problem (Part Three)
Let's imagine that we have a plan for reading other plans and saying if they will end.
Our imaginary plan is called E, for Ending. We want to know if a plan like E is possible.
We do not know what the steps of plan E are.
All we know is that we are imagining that we can follow plan E to read another plan and say whether it will end or not.
(We need a name for this other plan. We'll call it X.)
But wait! We know there are plans that sometimes end, and sometimes go on forever. Here is one —
Plan Z:
Plan Z will always stop if the number we start with is bigger than zero and is a whole number.
But if our number is one-half (or something else not whole) then Z will never end.
That is because our number will go right past zero without ever being zero.
Plan Z is not really whole by itself.
It needs something else that we give it: the number in step 1.
We can think of this number as "food" for the plan.
The "food" is something Z needs in order to go, or even to make sense.
Some food is good for you, and some is bad for you ...
... and whether Z ends or not depends on what number we feed it.
Plan Z ends if we feed it the number 1 or 42, but not if we feed it the number one-half.
And so when we ask "Will plan X end?" we really should ask "Will plan X end, if we feed F to plan X?"
So in order to follow plan E, we need to know two things: a plan X, and a something called F.
(What kind of something? Whatever kind X wants.
If X wants a number, then F is a number.
If X wants a cookie, then F is a cookie.
If X wants a plan to read, then F is a plan.)
Following E will then tell us if X-fed-with-F will end or run forever.
Now here is another plan —
Plan G:
So when we follow plan G, we don't do the same thing that X does.
We do the other thing!
If X never ends, then G ends.
And if X ends, then G never ends.
But what happens if X is G?
G is a plan, and G wants a plan for its food. So we can feed G to G itself!
If we feed G to G, then G will end if G doesn't end.
And G will go on forever if G ends.
That does not make any sense at all.
Everything about that makes no sense!
It is like saying "If the cat is white, then the cat is not white."
It is really wrong!
What part is wrong, though?
G is very simple. There is nothing wrong with G.
The wrongness is in the thing that we imagined:
Plan E, the plan that can tell us if any plan will end or not.
This means that E is not really possible.
That is the part that looked like it might make sense, but really it did not.
Oh well.
It sure would be nice if E was possible.
But it is not.
xkcd's Up-Goer Five comic gave technical specifications for the Saturn V rocket using only the 1,000 most common words in the English language.
This seemed to me and Briénne to be a really fun exercise, both for tabooing one's words and for communicating difficult concepts to laypeople. So why not make a game out of it? Pick any tough, important, or interesting argument or idea, and use this text editor to try to describe what you have in mind with extremely common words only.
This is challenging, so if you almost succeed and want to share your results, you can mark words where you had to cheat in *italics*. Bonus points if your explanation is actually useful for gaining a deeper understanding of the idea, or for teaching it, in the spirit of Gödel's Second Incompleteness Theorem Explained in Words of One Syllable.
As an example, here's my attempt to capture the five theses using only top-thousand words:
If you make a really strong computer and it is not very nice, you will not go to space today.
Other ideas to start with: agent, akrasia, Bayes' theorem, Bayesianism, CFAR, cognitive bias, consequentialism, deontology, effective altruism, Everett-style ('Many Worlds') interpretations of quantum mechanics, entropy, evolution, the Great Reductionist Thesis, halting problem, humanism, law of nature, LessWrong, logic, mathematics, the measurement problem, MIRI, Newcomb's problem, Newton's laws of motion, optimization, Pascal's wager, philosophy, preference, proof, rationality, religion, science, Shannon information, signaling, the simulation argument, singularity, sociopathy, the supernatural, superposition, time, timeless decision theory, transfinite numbers, Turing machine, utilitarianism, validity and soundness, virtue ethics, VNM-utility