O my Luve's like a red red rose / That's newly sprung in June

When you make an analogy, as you know, you’re pointing out a correspondence between two things — some way of mapping the salient features of A onto the salient features of B, and vice versa.

And sometimes, “the salient features” are kinda superficial (e.g. “he looks kinda like Shaq”; “a car is like a wagon pulled by a really strong horse”). But some analogies (my favorites) point out that two different systems work kinda the same way, so that you can apply your intuitive gears-level understanding of something you’re familiar with, to figure out how a seemingly unrelated system works.

And it turns out that certain analogies come up implausibly often. It’s like the universe only knows how to build a handful of distinct systems, and in a desperate attempt to keep you entertained it just dresses them up in a hundred different costumes. Consider:

  • “If I tweet this really hilarious image macro, and everybody who sees it retweets it to their followers, how many people will it reach?”

  • “What’s the shortest path through this maze?”

  • “If I get mono, and pass it onto the people I’m dating, and they pass it onto the people they’re dating, etc., how many people will wind up getting it because of me?”

  • “If Genghis Khan had some distinctive mutation on his Y-chromosome, how many people should I expect to have that mutation now?”

  • “If I discover a bug in this piece of code, and any code that calls buggy code might be buggy, do I need to worry whether the login-authentication code is affected?

Isn’t the symmetry, the correspondence, the isomorphism, the strength of analogy between all these problems beautiful? Like, you can transmute the mononucleosis problem into the retweeting problem by replacing “X dates Y” with “X follows Y”, and “has caught mono” with “has seen the macro,” and basically all of your intuitions from one problem apply to the other. They’re all the same shape on some really deep fundamental level:

  • Structure: “I have a bunch of things, and there are relationships between pairs of things (‘X [follows/connects to/dates/is a son of/calls] Y’), and we’re interested in some feature that ‘propagates’ between the things based on those relationships; starting from a single thing, where does that feature propagate to, or how many hops does it take?”

Also consider:

  • “If I get mono (again, jeez), and each person who gets mono typically gives it to 1–2 other people, how long until a thousand people have my strain of mono?”

  • “When an atom in this lump of uranium decays, the emitted neutrons typically hit a couple other atoms and cause them to decay too. After the first atom decays, how long until half of the atoms have decayed?”

  • “A nanobot can make a new nanobot in a couple hours. When I notice my skin is itchy because it’s being turned into nanobots, how long until the earth’s crust is entirely consumed?

  • Structure: “I have some process that feeds itself and accelerates through some kind of rough doubling mechanism; how long will it take to reach some critical point?”

Also consider:

  • “If this line dance shifts by two partners each iteration, will I ever get to dance with the cutie over there?”

  • “If I have a reading group on Mondays, and my spouse and I alternate which days we feed the dog, will I ever have to skip reading group to feed the dog?”

  • “If I use ‘Eeny, meeny, miny, moe’ to choose which of three children gets a candy, will I pick ‘Eeny,’ ‘Meeny,’ or ‘Miny’?”

  • Structure: “There’s some list of things (partners, days of the week, children) where we’re counting every nth, and when we get to the end of the list we start over; do we eventually hit every thing? Or how long will it take to do that? Or where will we be after some given number of steps?”

Also consider:

  • “This supermarket has like a million different kinds of meat substitute, and I don’t know how good any of them are, and it would take forever to try all of them; how many should I try before just deciding to stick with the best I’ve found so far?”

  • “Alex makes me really happy! But I’ve only ever dated, like, four people. I think Alex makes me the happiest of those four, but — should I still shop around more before settling down?”

  • “Great, once again it’s two weeks before the end of the semester and I haven’t started any of my three class projects. The classes have varying importances to me and I’m not sure how hard it’ll be to make progress on any of them, so I don’t know how to allocate my time most efficiently between them. Aaaaaaa — ”

  • Structure: “I have some limited resource (time, money, affection) and various options for how to spend it. I don’t have a very precise model of how much payoff I’ll get from investing in any given option. How do I invest my resource most efficiently?”

You might have noticed that those examples were about graph theory, exponential growth, modular arithmetic, and k-armed bandits.

You thought this was a post about literary and rhetorical devices? No. Math. Only ever math.

Whenever a huge pile of things share some underlying structure, there is some mathematical abstraction lurking there, ready to be teased out, solved, and applied to all hundred systems at once. I didn’t even give examples of all the situations that are analogous to arithmetic, because there are so many and the correspondence is so obvious.

Math, viewed through this lens, is the study of analogies.

Or a language for discussing analogies — neatly decoupled from the distracting details that most concrete systems exhibit.

Or a framework for constructing new analogies, by decomposing complex systems into simple ones for which you already have a firm intuition.

Or something you can do during your Literary Analysis and Composition class, now that you can link your teacher to an explanation of why it’s really the same thing.