Math must be presented for System 1 to absorb and only incidentally for System 2 to verify.

I finally have a sort-of formalizable guideline for teaching and writing math, and what it means to “understand” math. I’ve been unconsciously following this for years and only now managed to write down explicitly what it is that I’ve been doing.

(This post is written from a math-centric perspective, because that’s the domain where my concrete object-level examples from. But I suspect much of it applies to communicating hard ideas in general.)

## S1 and S2

The quote above refers to the System 1 and System 2 framework from *Thinking, Fast and Slow*. Roughly it divides the brain’s thoughts into two categories:

- S1 is the part of the brain characterized by fast, intuitive, automatic, instinctive, emotional responses, For example, when you read the text “2+2=?”, S1 tells you (without any effort) that this equals 4.
- S2 is the part of the brain characterized by slow, deliberative, effortful, logical responses; for example, S2 is used to count the number of words in this sentence.

(The link above gives some more examples.)

The premise of this post is that understanding math well is largely about having the concept resonate with your S1, rather than your S2. For example, let’s take groups from abstract algebra. Then I claim that

is a group under the usual multiplication. Now, if you have a student who’s learning group theory for the first time, the only way they could see this is a group is to compare it against a list of the group axioms, and have their S2 verify them one by one. But experienced people don’t do this: their S1 automatically tells them that “feels” like a group (because e.g. it’s closed and doesn’t have division-by-zero issues).

I think this S1-level understanding is what it means to “get it”. Verifying a solution to a hard olympiad problem by having S2 check each individual step is straightforward in principle, albeit time-consuming. The tricky part is to get this solution to resonate with S1. Hence my advice to never read a solution line by line.

## Writing for S1

What this means is that if you’re trying to teach someone an idea, then you should be focusing on trying to get their S1 to grasp it, rather than just their S2. For example, in math it’s not enough to just give a sequence of logical steps which implies the result: *give it life*.

Here are some examples of ways I (try to) do this.

First, **giving good concrete examples**. S1 reacts well when it “sees” a concrete object like above, and can see some intuitive properties about it right away. Abstract “symbol-pushing” is usually left to S2 instead.

Similarly, **drawing pictures**, so your S1 can *actually* see the object. On one extreme end, you can *write* something like “a point $S$ lies on the polar of $T$ if and only if $T$ lies on the polar of $S$”, but it’s much better to just have a picture:

You can even do this for things that aren’t really geometrical in nature. For example, my Napkin features the following picture of cardinal collapse when forcing.

Third, **write like you talk**, and share your feelings. S1 is emotional. S1 wants to know that compactness is a *good* property for a space to have, or that non-Noetherian rings are *way too big* and “only weirdos care about non-Noetherian rings” (just kidding!), or that ramified primes are the “finitely many edge cases” and aren’t worth worrying about. These S1 reactions you get are the things you want to pass on. In particular, avoid standard formal college-textbook-bleed-your-eyes-dry-in-boredom style. (To be fair, not all textbooks do this; this is one reason why I like Pugh’s book so much, for example.)

Even the mechanics on the page can be made to accommodate S1 in this way. S1 can’t read a wall of text; S2 has to put in effort to do that. But S1 can pick out section headers, or **bolded phrases like this one**, and so on and so forth. That’s why in Napkin all the examples are in separate red boxes and all the big theorems are in blue boxes, and important philosophical points are typeset in bold centered green text. This way S1 naturally puts its attention there.

## But do not force it

On the flip side, if you’re trying to *learn* something, there’s a common failure mode where you try to keep forcing S2 to do something unnatural (rather than trying to have S1 figure it out). This is the kind of thing when you don’t understand what the Chinese Remainder Theorem is trying to say, so you try to fix this by repeatedly reading the proof line by line, and still not really understanding what is going on. Usually this ends up in S2 getting tired and not actually reading the proof after the third or fourth iteration.

(For the Chinese remainder theorem the right thing to do is ask yourself why any arithmetic progression with common difference 7 must contain multiples of 3: credits to Dominic Yeo again for that. I’m not actually sure what you’re supposed to do when stuck on math in general. Usually I just ask my friends what is going on, or give up for now and come back later.)

Actually, I really like the advice that SSC mentions: “develop instincts, then use them”.

Amazing of you to mention this. I always thought you were one of the ones who did this the best – which is why I love reading the Napkin.

I just hope more people would understand this philosophy. There are so many lectures in which professors just keep on going in S2 style, wasting time on meaningless, boring computations without ever saying a word in S1 style. And I absolutely hate that. Give us some intuition/motivation!

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Do you have any recommendations for S1-style textbooks that cover the standard undergrad curriculum?

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I agree that inserting lots of “S1 style” elements is very helpful when teaching/being taught, but I think it’s less effective in textbooks. From my experience, as a learner, I only really develop these intuitions if I’m work through the statements to arrive at these core ideas on my own. I think the difference is that with a teacher you can ask questions and juggle wrong ideas and intuitions around until you figure them out, whereas in a textbook it’s simply laid out for you. I think from an individual perspective these S1 elements are more important the less opportunities you have to ask other people about the material.

I suppose Ravi Vakil’s Rising Sea and, to a lesser extent, Hatcher’s Algebraic Topology are the most famous examples of S1-style textbooks. They are easy to read, but almost absurdly long…

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