People often ask me how they can guarantee they won’t forget anything they’ve learned. But I think forgetting isn’t such a bad thing and that trying to avoid it completely is a loser’s strategy.
Trying for a perfect memory is a defensive strategy, it’s protecting the stockpile of knowledge you possess against atrophy. In theory it sounds good, who wouldn’t want to forget less and remember more? The problem is opportunity cost. The time you spend memorizing and making sure you never forget is time you could have spent learning something new.
Even within a specialization, the knowledge you possess is probably only a tiny fraction of all useful things you could know. Given that you know so little, playing defense seems like a poor strategy.
Learning vs. Relearning
Relearning is generally much easier than learning for the first time. Knowledge fades but it is rarely completely deleted, and I’d wager even for topics which have faded completely from conscious access there would still be a learning advantage for relearners.
The exception to this is for knowledge which was never really learned the first time. I often hear students complain about knowledge they’ve completely forgotten from prior classes, but my guess is that they never really understood those ideas.
Is Spaced Repetition Worth It?
A technique that’s been getting a lot of press is the idea of spaced repetition. It’s the idea that the best way to form lasting memories is to remind yourself of an idea right before you would forget it. Spaced repetition systems like Anki  have been popular for flashcard style review.
I’m skeptical of the value of an SRS for most domains of knowledge. The problem is simply that just because you’re reminding yourself of an idea doesn’t make it useful. Useful and important ideas recur frequently, so spaced repetition is naturally built into the process of learning aggressively.
People have asked me if I have a mechanism to review material from courses I completed earlier in the MIT Challenge. Initially, I had considered creating one, but going through the classes has shown me that it isn’t necessary.
For any given class, some ideas will be very useful and important, others less so. If an idea is useful, it shows up in more than just one class. Huffman coding has probably shown up in 4-5 classes I’ve done, so I’m getting tons of repetition even though I never use a formal system to remind myself.
Even languages, the favorite child of SRS seem amenable to this approach. If you spend most your time actually communicating, the words and phrases you memorize are precisely the ones that come up most frequently. Perhaps just going out and speaking a lot  is the best kind of spaced repetition.
The same is true in non-academic learning. When I read a book, I try to deeply understand it, but I don’t make any system to guarantee that knowledge is perfectly preserved. The reason is that I know if the ideas are useful and important, they will show up as themes in other books.
The solution to spaced repetition, therefore, probably isn’t looking back and reviewing, but going forward aggressively. Expanding your knowledge instead of fortifying it.
My strategy is aggressive learning. This is the idea that my knowledge is far less than 1% of the total human knowledge available, and probably less than 10% for the things I’ve chosen to specialize. Given these enormous gaps in my understanding, it makes more sense to learn aggressively than worry about forgetting.
If the brain truly has a maximum capacity, I doubt if any person has ever reached it. Knowledge seems to be stored on top of each other, so even if some ideas are buried under new ones, they still form part of the foundation.
Aggressive learning doesn’t mean superficial learning. By all means, when you learn an idea, learn it well. My biggest takeaway after doing this MIT Challenge was the importance of writing exams and practice problems to learn ideas deeply, it’s too easy to skip the tough stuff and never really get the ideas you’re trying to learn.
When Should You Look Back?
Often the paths and trails into unexplored territory circle back to familiar ground. This presents an opportunity to relearn those old ideas or to view them in a new way and get a better connected understanding of them.
At the beginning of the MIT Challenge  I took a class which introduced the Laplace Transform and, honestly, I didn’t get it. I could manipulate them well enough to pass a test, but I couldn’t see what they were or why they were useful.
However, because they were an important idea, they showed up again and again. Each time I circled back I got another chance to learn them more deeply. Had they not been important, I might never have seen them again and they would have faded from memory, but if they aren’t too important, why would that matter?
Aggressive learning works because it circles back and reminds you of precisely the ideas that are more prolific, useful and important.
Why Don’t People Learn Aggressively?
The attitude of aggressive learning and curiosity is forced out of students by education. Because almost all classes have a closed curriculum, where only a certain set of knowledge is tested, it rewards being able to get really good coverage of some arbitrary subsection.
But life doesn’t have a finite set of readings or ideas to understand. Success in the real world means knowing lots of things and being able to learn the answers to questions that won’t show up on any test.