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:
- Things to be understood.
- 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:
- 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.
- 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.
- 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.