# Introduction to Introduction to Category Theory

post by countedblessings · 2019-10-06T14:43:46.977Z · score: 117 (50 votes) · LW · GW · 12 commentsCategory theory is so general in its application that it really feels like everyone, even non-mathematicians, ought to at least conceptually grok that it exists, like how everyone ought to understand the idea of the laws of physics even if they don't know what those laws *are*.

We expect educated people to know that the Earth is round and the Sun is big, even though those facts don't have any direct relevance to the lives of most people. I think people should know about Yoneda and adjunction in at least the same broad way people are aware of the existence and use of calculus.

But no one outside of mathematics and maybe programming/data science has heard of category theory, and I think a big part of that is because all of the examples in textbooks assume you already know what Sierpinski spaces and abelian groups are.

That is to say: all expositions of category theory assume you know math.

Which makes sense. Category theory is the mathematics *of math*. Trying to learn category theory without having most of an undergraduate education in math already under your belt is like trying to study Newton's laws without having ever seen an apple fall from a tree. You *can*...you're just going to have absolutely no intuition to rely on.

Category theory generalizes the things you do in the various fields of mathematics, just like how Newton's laws generalize the things you do when you toss a rock or push yourself off the ground. Except really, category theory generalizes what you do *when you generalize with Newton's laws*. Category theory *generalizes generalizing*.

Therefore, without knowing about any *specific generalizations*, like algebra or topology, it's hard to understand *general generalities*—which are categories.

As a result, there are no category theory texts (that I know of) that teach category theory to the educated and intelligent but mathematically ignorant person.

Which is a shame, because you totally can.

Sure, if you've never learned topology, plenty of standard examples will fly over your head. But every educated person has encountered the idea of generalization, and they've seen generalizations of generalizations. In fact, category theory is very intuitive, and I don't think it necessarily benefits from relating it all as quickly as possible to more familiar fields of mathematics. Instead, you should grasp the flow of category theory itself, as its own field.

So this is (tentatively, hopefully, unless I get busy, bored, or it just doesn't work out) a series on the basics of category theory *without assuming you know any math*. I'm thinking specifically of high school seniors.

There is no schedule for the posts. They'll just be up whenever I make them.

Why category theory? And why lesswrong?

Well, category theory is a super-general theory of everything. Rationality is also a super-general theory of everything. In fact, we'll see how category theory tells us a lot about what rationality really is, in a certain rigorous sense.

Basically...rationality comes from noticing certain general laws that seem to emerge every time you try to do something the "right" way. After a while, instead of focusing so much on the specifics, it starts being worth it to take a step back and study the general rules that seem to be emerging. And you start to notice that doing things the "right" way gets a lot easier when you *start* with the general rules and simply fill in the specifics, like how the quadratic formula makes quadratic equations a cinch to solve.

Category theory gives us *all* the general rules for doing things the "right" way. [LW · GW]

(Don't actually hold me to demonstrating this claim.)

Why should *you* be interested in category theory?

One is because category theory is going to rise in importance in the future. It offers powerful new ways of doing math and science. So get started!

Two is that category theory makes it much easier to learn the rest of math. Well, *maybe*—this is an experiment, and a big motivation for doing this. How fast and well do people learn regular math if they can just say, "Oh, it's an adjunction" every time they learn a new concept?

Three is that referencing homotopy type theory in conversation will make you sound cool and mysterious.

Please let me know if there's any interest in this.

## 12 comments

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I'm still buying the CT hype, so very interested to see more of this. However, I've been buying the hype for some 10+ years now and trying to learn CT on and off, and still can't point to a single instance of being able to use it either to approach a problem or understand something better, so I'm pretty skeptical about this being teachable to a mathematically naive audience in a way that they can internalize much anything about it that's both correct and usable in some practice that isn't advanced math study.

When we get to adjunction, I'm hoping the utility of the series will start to become clear.

This sounds like a very exciting project and a solution to an open exposition problem.

I look forward to reading the posts!

I also wonder if you know what kinds of things would motivate you to write them?

Interesting discussion? Getting data about your ability to tutor concepts? Money (maybe we could organize a Patreon with interested LW users if so)?

I'm motivated by the essays themselves. I believe this material should exist. It's also good for my own understanding to write them.

While I share your enthusiasm toward categories, I find suspicious the claim that CT is the correct framework from which to understand rationality. Around here, it's mainly equated with Bayesian Probability, and the categorial grasp of probability or even measure is less than impressive. The most interesting fact I've been able to dig up is that the Giry monad is the codensity monad of the inclusion of convex spaces into measure spaces, hardly an illuminating fact (basically a convoluted way of saying that probabilities are the most general ways of forming convex combinations out of measures).

I've searched and searched for categorial answers or hints about the problem of extending probabilities to other kinds of logic (or even simply extending it to classical predicate logic), but so far I've had no luck.

I am strongly interested. I've been trying to get some real math under my belt, but to be frank it isn't my talent so I am perpetually on the lookout for exposition and exposition accessories.

I've tried to learn the basics of the category theory some years ago, already having some background in algebraic topology, mathematical physics and programming. And, presumably, in rationality. I got the glimpses of how interesting it is, how it could be useful, but was never quite able to make use of it. Very curious if your series of posts can change that for me. Keep going!

I apologize for spamming with two posts on the same day about the same content, but I actually wrote this post first. It was waiting to be moderated for several days, so I wrote the other one and submitted it, and to my surprise it was immediately published. I guessed this one must have gotten stuck in a queue or something, so I rewrote it and published it. My bad, we'll definitely go one at a time from now on.

But in a way it's fitting, as having *two* introductory posts should definitely indicate just how slow the pace of this series is going to be.

I am looking forward to this. I did math olympiads at high school, then switched to computer science, and then spent a few decades writing apps that read values from database, display them in HTML, and store the edited values again in the database (always in some new framework, so I have to keep running and yet remain at the same place). Sometimes I wonder where the alternative paths could have lead. It seems that my brain is still capable of understanding math; I can read about new topics and develop some intuition about them (and verify it with people who actually understand the stuff). Understanding the category theory would... well, feel good; as if I am getting a glimpse into the alternative universe where I continued doing math.

Sometimes things are explained in an unnecessarily difficult way. I understand that if you are a university professor teaching X, and you know that *all *your students learned Y during the previous year, it makes sense to write a textbook of X assuming deep and fresh knowledge of Y. But because of job and kids, I don't have the time to walk the exact path of a university student, so I would appreciate shortcuts.

yes, please! ... Eugenia Cheng's book "**Cakes, Custard and Category Theory**" nibbles around the edge ... perhaps you can prepare the main course? (^_^) ...

My masters degree involved a good bit of category theory. Personally, I don't see how it has any use outside of mathematics. (Note 'maths' includes 'mathematical logic' - so it's still a broad field of applicability).

I am highly motivated to be persuaded otherwise, and hence will be watching this series of posts with keen interest.

---

<disclaimer>

I am not a working mathematician, and have not published any papers. My masters thesis involved a lot of category theory - but only relatively simple category-theoretic concepts (it was an application of category theory to a subfield of mathematical logic).

Limits, free objects, adjunctions, natural transformations etc. but not higher-order categories, topoi/toposes or anything fancy like that.

</disclaimer>

<handwavy discussion of technical math>

As I understand it, the usual application of category theory is mostly to things involving natural transformations (it is said the need for a way to formalize natural transformations is what led to the invention of category theory) - and even then, it seems mostly to be applied to nice algebraic objects with (category theoretic) limits, and slight generalizations of these. So, to groups and rings and modules, and then to some categories made of stuff kinda like those things.

There's also the connection to topology and logic, via simplicial complexes, homotopies, toposes, type theory etc. which seems very interesting to me. It seems useful if you want to think about constructive mathematics (i.e., no law of excluded middle) - which is promising for maths involving some notion of 'computation' (for a given abstraction of computation) which has obvious applications to computing (especially automated theorem provers / checkers).

In these senses, I can certainly see its use as another mathematical field, and a good way of reasoning IN MATHS or ABOUT MATHS. But I don't quite understand its tremendous reputation as this amazing mathematical device, and less so its applications outside of what I've mentioned above.

</handwavy discussion of technical math>

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In particular, from my admittedly limited knowledge, category theory only seems useful:

a) If you already have a bunch of different fields and you want to find the connections.

b) If you want to start up a new field, and you need a grounding (after which the useful stuff will be specifically in the field being developed, and not a general category theory result).

c) For good notation / diagrams / concepts for a few things.

EDIT: Interested to hear the opinion of someone who actually works with category theory on a regular basis.