Rethinking “Learn it Once”

When I wrote Learn More, Study Less in 2008, one of the big pieces of advice was to “learn it once”. The main idea being that, while review is still necessary, you shouldn’t procrastinate on what you’re learning—if you don’t understand something, the pre-exam cram session isn’t the time to learn it.

I stand by that original intention and I still believe that, given the classroom environment where ideas are presented in a structured way, it’s an appropriate maxim.

However, another interpretation I hadn’t really intended was the idea that you should only require one pass of an idea to deeply understand it. “Learn it once” has meant, for some, “never learn it again.”

This second interpretation bothers me. First, it’s almost certainly false. Deep understandings almost always take a couple passes. That doesn’t mean you should postpone developing insight—repetition without understanding is just memorization. But it does mean that “learn it once”, taken in this context, is bad advice.

Is Repetition a Good Thing?

I’ve had mixed feelings about repetition being used as a learning tool. I’ve seen too many students memorize things that need to be understood. Even if you did need only memorized information and zero understanding, creating connections and using vivid mnemonics makes the same process faster.

But, provided blindly repeating information isn’t your first action, repetition isn’t bad. Most of the research shows that repeated viewings of the same idea strengthen memory. Taking the spacing effect into account, learning something multiple times might even be better than trying to cluster it in one session.

A better, albeit more wordy, maxim might be “Don’t procrastinate on learning ideas as you’re exposed to them. Build insights and connections before memorizing anything.” Doesn’t have the same ring as “learn it once” but it’s a little more nuanced and accurate.

If I were to implement this advice as a student, I would still do all the things I mentioned in Learn More, Study Less. Create metaphors, explain ideas as if you were teaching them to someone else, use mnemonics for fickle details. But I’d also add active recall, as an important method for strengthening long-term memory.

Peeling the Onion

Most learning tasks inevitably break down to one of two groups:

  1. Things to be understood.
  2. Things to be remembered.

Most items fit somewhat in both categories, but rarely equally. Physics has some memorization, but answering problems depends crucially on having insight. Languages have some insight, but they’re mostly memory and practice.

A common mistake, and hence a theme of my writing, is to assume something requires mostly memory, when it actually requires mostly understanding. I die a little inside each time I see a student memorize a formula without trying at all to figure out how it works.

Repetition for things that need remembering is obviously useful. Holistic learning is still important, because memory is associative, so connections between ideas will improve recall. But even a well-tuned mnemonic system will still benefit from repetition for purely arbitrary information.

Repetition for concepts can be dangerous, if only because it can sometimes create the illusion that you understand it. When I was doing MIT classes, I had to watch myself that I wasn’t merely memorizing solution patterns, since they gave the feeling of progress but would collapse whenever a new type of question was introduced.

However, provided you are actively building insight, repetition can also be useful for understanding. I see understanding most concepts like peeling an onion. Each time you explore it, you have the opportunity to look at it in a new way. Repeated exposures allow you to burrow down to deeper layers of understanding.

I felt this way with learning many mathematical concepts. The depth of insight in something like Fourier analysis is so large it’s impossible to fully grasp it with a first pass. New exposures, particularly those from different vantage points, reveal new insights each time.

This is true even of ideas you may have already “learned”. A simple idea like addition can be peeled all the way to Zermelo-Fraenkel set theory. Having learned something once here is misleading, because learning is inevitably incomplete.

Practical Advice for Self-Learners and Students

What does this mean for you? For the most part, I don’t believe it changes the implications of what I’ve been advising all along. Don’t procrastinate on understanding. Never substitute memorization for insight. Use connections and mnemonics to remember ideas with less effort.

However, I will explain how this has manifested itself in my own learning so you can see how it might affect yours:

  1. Almost all learning is incomplete. Circling back to gain new insights or refresh your memory of old ideas is a part of learning. I do feel sometimes self-learners are too conservative in their exploration of new knowledge, but this doesn’t mean learning something is a box you can tick and never return to.
  2. Repetition, provided it supplements holistic learning, can be a good tool. In the past I’ve been hard on spaced repetition systems, but now I see that they can also be a way of automating the reminder process. Supplement, don’t substitute.
  3. Use active recall. Active recall is where you try to answer a question, without seeing the solution. This is superior to passive recall where you see the question and answer at the same time.

  • Alejandro

    Scott, there’s a section on the book “Secrets of a Buccaneer Scholar” that talks about alternation and cyclic learning. It resonates a lot with this. I deal mostly with “things to be understood” (I’m a developer) and I can rarely understand concepts on the first take. I may develop some understanding of it, but not the type of understanding that renders the concept most useful.

    Thanks for the post, it brought back the idea into mind 😉

  • Sexy Beast

    Man … this blows my mind … I was going for your advice from Learn more study less …. and now you contradict yourself a little bit . I mean c’mon … you must be kidding me !!

  • Winston

    Thanks for the clarification. I was thinking along those lines too when I read the LIO principle.

  • Christoph

    I think repetition is equally essential as holistic learning. Because forgetting exists, yes really =) (….
    Scott, you studied economics, so you will maybe familiar with the concept, that a strategy is as good as its implementation: strategy * implementation = result.
    In my opinion the mathematical relation between holistic learning and repetition(=>retention rate) is like in the strategy example a multiplication sign: holistic learning * retention rate = result (For me holistic learnings focuses on learning/understanding and this very well =))
    Your will argue: no with holistic learning you will get a “high” retention rate =)
    Nope you will get a “higher” rentention rate and the semester is about 15 weeks ( maybe +1 prep week and then semester exams) long, so you WILL forget a lot if you dont repeat,for me it doesnt matter if you use metaphors, mnemonics etc., time to time you need to refresh/think about the metaphors, mnemonics etc. you used to learn.
    Also it depends how your courses are structered: As an example the different examination procedures as “I” see them in US(midterms and semester exams, quizzes, projects etc.) and EU (projects, semester exams). Maybe you have annual exams, as example ETH Zürich “Basisprüfung” failure rate ~40% of all first year students. If you have midterms/quizzes you’re forced to repeat during the semester, if not then you end in cramming/relearning in the pre-exam week(s), maybe you’re cramming/relearning during pre-midterms weeks(s). I hope you see my point. (maybe the wording of my arguments are too vague, english is not my native language)

  • Jan

    This was actually always a point that I thought I didnt agree on with you.
    However in light of your clarification I have to say you are right.
    There is an interesting theory by Objectivist philosopher Leonard Peikoff called the spiral theory of knowledge:

    The spiral theory [of knowledge] is simply this idea: first we learn a given idea; then we leave it — we move on to something else, we learn other subjects. Then we encounter the original idea again, but now with more knowledge, with a deeper context.

    [Extemporaneous, unedited formulation. From Leonard Peikoff, “Understanding Objectivism” lecture series (1984), Lecture 3.]

    This theory basically says that the first time you encounter something, you cannot learn everything about it, because some things about it are in relation to things you don’t yet know. The more you learn, the greater your potential understanding is of topics you’ve already visited. So on revisiting them, you learn new things about them, gaining a deeper, more tightly integrated understanding of them, and of the things they relate to. It’s because understanding is integration.

    Everyone has probably experienced this, as your context grows and changes things you learned before can be more tightly integrated more deeply understood in that context.
    This is also one of the reason memorization is such as bad policy, it is on the perceptual almost sensual level of signs and auditory sounds and there is not way of integrated it, since there is no meaning assigned to the material memorized. Another technical term for this is floating abstraction, you are unable to tie it down to concretes and further knowledge and experience can to nothing to corroborate the abstraction.

    I am sorry for somewhat highjacking your post. I just think it is fascinating to look at this coming from epistemology and the relation of human knowledge to existence, for ultimately the proper relation of the two determines the best way to learn things.

  • Nate Anglin

    As Jan mentioned, it’s difficult to learn something entirely upon the first view and must be revisited to encourage retention. But what do you think about putting what you first learn immediately into practice? I know when I apply what I just learned my retention rate skyrockets.

  • Franklin Chen

    It depends on what “it” means. I think that’s where the misunderstanding lies, regarding to learning something once or more than once. There is no “it”. There is depth in fundamentals that one simply does not fully grasp or even see at first. So rather than “learn more than once”, I’d rather frame the situation as “learn what you didn’t see or didn’t connect before”.

  • Oliver Powell

    I definitely agree that it often takes multiple passes at something to truly understand it. However, I also feel that ultimately any knowledge you want to remember needs to be memorized, including the understanding of something. I had an 86% average in first year electrical engineering (at technikon level), and this year I made dean’s merit list in the same field at the top university in Africa. (UCT).

    I made sure to understand most things as deeply as I could and develop as much insight as possible. If you had to ask me to rewrite those exams now I would do horribly, despite my understanding. This is not because I didn’t really understand it, but rather because if you don’t make an effort to remember what you understand, you will just forget it, unless you are exposed it regularly.

    I highly recommend using spaced repetition software for this. I understand your take on it is, “if it’s important it will show up often”, but i’d rather just remember what I want to. My only rule is that I only memorize what I understand.

    I’m using it for increasing programming knowledge and productivity and its working wonders.

  • Scott Young


    I think that was what I tried to rally against as much as possible. People thinking that “oh, I’ll learn this later, even though I don’t really understand it (or haven’t really built any connections into it)”. Then you’re not really preparing your brain to understand it later.

    To all,

    Learn it once (as interpreted as I describe in the introduction) is still a good idea. It is based on my experience taking structured courses where I can honestly admit, one of the best techniques for reducing studying time is to make strong connections between ideas so that the burden of review is minimal.

    I wrote this post because of how often I have seen it being an excuse to not review, not the imperative to not delay learning as I’d hoped.


  • Martin

    Human will always tend to find a comfort zone in every area of their life, will always “hacks” even the safest procedure and approach for something, into their own comfort as much as possible. There are always at least two point of view for something in life, which both has to be taken of. I found that we cannot find one “ultimate” solution for something, and strike full force at that direction without rethinking “what will be happen if I run to the opposite side?” because at some point human’s mind will hacks into the solution which currently “bother” their comfort, and find that tiny little spot of comfort zone inside the solution which will renders that “ultimate” solution to be not-so-ultimate anymore for that particular person. We need balance for almost everything in life, and that balance point can be different between every individuals. Solutions give us reason to going forward or change to into a better shape. Excuses, on the other hand, give us reason to stay in place, no matter how “good” the excuses is, or how “similar” the excuses to the original solution from which it “evolves” upon. An excuse is an excuse, and must be eliminated at all cost from human race if we want to be a better sentient race.

    I’ve been always inspired by your insight, and this post is neither less. Thank you for drawing the line where your solution will be used as an excuse, not a solution anymore.

  • Trishna Sharma

    Great post, Scott. Although, I never really had a problem understanding this point 🙂

    Sexy Beast – It’s not really a contradiction. I think what Scott is saying is that learning really can take only one time. But, as humans we tend to forget things over time which is where repetition and memory tools come into play. It is not so much that you have to RE-learn something. You just need to make sure you remember it over the long-term and see how it connects to the bigger picture.

    If you were to choose not to add in some repetition and you forgot almost everything you learned, the second time learning it would be nowhere near as difficult or as time-consuming as the first. Once you started getting into the subject again, you would start to remember things and the pieces would fit back together. So really, you are only having to learn something once. Repetition is what keeps the information solidified in your mind.

  • Jeff

    Being a self-taught programmer and guitar player, I can say that I haven’t always learned things in the right way…I usually do something repetitively to memorize it, without always understanding what it is I’m actually doing.

    Understanding and learning scales before learning to play a song is much more valuable then going straight to playing. When you understand what you’re playing you can pick new songs up quicker because you can “guess” where notes fall and you have a better “ear” for picking the right notes, when you just memorize fingerings you aren’t really learning anything.

  • dag

    I see eye to eye with you. All what you say worked for me in the past.

    However, I noted something peculiar. The advices you give in the very end of the article, ‘active recall’ over all, are difficult to follow. When I did so I was able to really study less and learn more (apart from having better marks), but I could take advantage of them only because I was very motivated. If I hadn’t been, I think I wouldn’t have such a willpower. So, what about taking projects when you’re not that motivated?

  • SBC

    New reader to the blog. 🙂

    I was curious. Did you ever used Mind Mapping as well for the learning process?

  • eduardo_spain

    First of all, I say sorry for my english, it is not quite fluent. It is good to grow and progress, looking for feedback and changing what is no efficient. Related to the topic, there is a software that is really helpful ( ). Apart from other softwares, this has the “incremental reading” option, and it is a really useful tool. Combining the Cognitive Load Theory and the testing effect, among others, it is the perfect tool for people who wants to learn quickly, to forget little, and to integrate knowledge and boost creativity. Give it a try 😉

  • Robin

    I have a doubt about repeating older stuff we’ve learned . Is it possible to remember everything we ever learn ? Including names , dates , numbers and little details ? I mean , a few months after high school I could’t remember a lot of stuff . I’m sure I won’t remember quite a lot of things a year or so after college . Not to mention all the facts I’ve read from all the many books , wikipedia articles and random sites etc

    My question is , is it practical to revise everything we have learned ? Isn’t the magnitude of everything we have learned too much ? Even if we could indeed revise everything , won’t we just be spending all the time we saved by studying efficiently on revising ?