No matter who you talk to, students or professors, they all say that the materials they once learned can be quickly forgotten without further experiences that utilizes or enhances on that prior experience.

I fondly remember a professor once telling me that his final year in undergrad was the happiest year in his life, at time when he felt he could do anything, from nuclear physics to parallel programming to electromicroscopy to constructing an audio amplifier...then one year later he has forgotten almost everything.

This is more noticeable in students, where it is often exaggerated to the cliched phrase "you never actually use anything you learn in school".

This is problematic for students who aspire for higher learning because much of the material or understanding is accumulative. It amazes me how people manage to get all the way to the top of the academic ladder (PhD, Post-Doc) without losing previously gained knowledge along the way. I'm sure there were important theorems, relations or techniques that I once learnt, maybe was even an expert in, the question is how do I unlock these memories so I can be more effective in tackling the problems I have today?

How do people deal with the inevitable loss of knowledge from years of disuse? Do you start from scratch? Can someone offer good ways for retention of class room materials?

  • 20
    Teach it (even if informally). Feb 9, 2015 at 17:35
  • 2
    There is no way to give a general answer to this question, because different subjects are different. Learning a foreign language has one set of issues. Learning Newton's laws of motion has a completely different set. Even within one subject there can be different approaches. I could memorize the names and dates of all the battles in the US Civil War, or I could look at how the war changed society, law, and government.
    – user1482
    Feb 9, 2015 at 20:57
  • 2
    Not only subjects are different, but people are also different. They learn in different ways and remember in different ways.
    – gnasher729
    Feb 10, 2015 at 11:13
  • 7
    If you forget it, just learn it again. It happens a lot quicker the second time.
    – N. Virgo
    Feb 10, 2015 at 15:24
  • 1
    Anecdotal: Bit I agree with @Nathaniel - once I've learned something once and studied the area around it, I can usually rapidly pick things back up even after several years. Taking a couple of days to get back up to speed is usually expected on a new project, even if you've not forgotten important material.
    – Jon Story
    Feb 10, 2015 at 17:02

6 Answers 6


This is problematic for students who aspire for higher learning because much of the material or understanding is accumulative.

Actually, that's precisely why it's not problematic. If you had to memorize random trivia, it would become more and more difficult as you had to remember more unrelated things, but academic studies work in the opposite way. Because it's accumulative, you are constantly practicing and applying what you learned before, which helps you solidify your understanding. Furthermore, the more you learn, the more connections you can see, and these relationships help you organize and retain knowledge. Of course you'll still forget some details, but you'll forget less than you expect and recover it more easily.

My impression is that severe forgetfulness in students typically occurs when they are studying ideas in isolation and setting them aside as soon as the class is over. Instead, it's important to play with ideas constantly. How are they related to your past studies? To other interests of yours? Can you think of further applications or connections? This can help bridge the gaps between how you learned these ideas. This sort of exploration is almost essential if you want to do research, and it's a useful study technique in any case.

  • 2
    To add: it is easy to forget things that you never need to use in years - but then, is it any bad? Feb 10, 2015 at 10:42
  • 1
    After exercising: "Those muscles hurt because you haven't used them in years" - If I haven't used them in years, why am I bothering to exercise them? The human brain is designed in the same way, but recovers much faster.
    – Jon Story
    Feb 10, 2015 at 17:04
  • @JonStory, Citation Needed.
    – Pacerier
    Feb 11, 2015 at 15:16
  • 1
    @Pacerier I'd call it assumed knowledge, unless I'm a significant outlier. Example: I didn't touch a rubiks cube for 4 years, couldn't remember how to do it and had to work it out again. I figured it out in a couple of hours as compared to the several weeks it originally took me. We forget specifics, but we retain a certain amount of knowledge: is that not obvious? Perhaps it's just me
    – Jon Story
    Feb 11, 2015 at 15:18
  • @PiotrMigdal, Of course it's bad. How could it be any good? Time is being wasted.
    – Pacerier
    Feb 12, 2015 at 10:06

I think that there are two fundamentally different types of learned materials to think about here, one that really matters and one that does not matter so much:

  • First, there is deep knowledge, which is the foundations of a subject, such as the principles of abstraction and recursion in programming, or asymptotic analysis in algorithms.
  • Second, there is the more surface knowledge of formulas and methods, common facts, artifacts, and patterns by which the foundational core is put to use, such as particulars of Java programming or the difference between Shell sort and radix sort.

The second is rapidly lost, while once you have grasped the first, I think that it never really fades, just gets a bit rusty. Many students, however, never really grasp the deep knowledge of a subject while taking it. You can often even get an A in a class just by brute force in drilling yourself on the surface knowledge of the subject. Worse yet, many students are encouraged to do this by a primary and high school education that intentionally focuses on "facts and dates" rather than principles.

I don't think that you need to worry about losing the surface knowledge of a subject. Surface knowledge is just the "working set" that you're currently most familiar with, and it changes all the time. More to the point, you can re-acquire lost surface knowledge quite quickly, if you know where the gap in your knowledge is (which deep knowledge will help you with). In fact, you should expect to lose surface knowledge quickly about anything you aren't actively using (one might think of it like a computer's cache).

Acquiring deep knowledge, on the other hand, affects how you see the world. As long as you lead an active intellectual life of any sort, you will somewhat see the world through the filters of the deep knowledge that you have acquired, and in doing so you will keep exercising it and retaining it. For example: anybody who has ever really understood basic physics will always have conservation laws popping up in their head; anybody who has ever really understood basic chemistry will keep noticing things about crystal structures and states of matter. And it goes for more advanced subjects too: for example, anybody who has ever really understood computational image processing will be influenced in how they think about Facebook image tagging and their camera and generally the images they see.

In short: don't worry about forgetting facts, and any subject that you truly completely forget is one that you never really understood in the first place.


I come from a programming background, so maybe my answer is not applicable here, but I think a very good way to retain knowledge is to teach it. I have started teaching some kids and older ones that were interested in the subject and I think it helped me a lot retaining the knowledge, plus they loved it!


One option would be to use a spaced repetition system. This would mostly be useful for facts, but you might be able to use it for processes too if you can phrase it as a question and answer.

Spaced repetition systems are essentially a computerised flash card database. These help you avoid the problem of a paper system: spending too much time on facts you remember easily and not enough on facts you forget, because the process of recalling a memory actually impacts our memories. To transfer data from the short term to long term memory you need to remember the data at a frequency somewhere between those extremes.

The difference from paper flash cards is that a database can store information for each card, such as a difficulty level and the time of the last card review. Using some complex maths I haven't looked into, the system will predict when you will forget each card and only ask you to remember it at that time. For some cards that could be every couple of years, for others it will be every day. With regular use you can maintain a database of tens of thousands of facts with only 5-10 minutes of practice each day.

One popular and free spaced repetition system is called Anki. It allows you to use multimedia in your cards, which could be very helpful depending on which memories you want to retain.

  • I doubt there's any complex maths going on. It's probably some dead simple algorithm + some randomness mixed in.
    – Pacerier
    Feb 12, 2015 at 10:08

1) By repetition

If you really would like to keep the knowledge you have, you have to repeatedly apply,read or learn it. E.g. write small cards with a question on the one side and the answer on the other and store them on your toilet or use some apps to do so. If you put a poster on your toilet like this one describing a cell signaling pathway and study it every day for about one or two years, you will remember it quite a long time.

2) By emotions

I can remember things pretty well, if I had some emotional moments while learning. Unfortunately that rarely happened with the stuff for university.

3) How to deal with the loss of knowledge ? I know where to look it up. Once you got the concept of something you will remember where to find the details.

  • Interesting perspective. I like the emotional bit but it is sort of a double edged sword. A partner who I was assigned to do assignments with actually passed away at the tail end of a very difficult course...I went into a whole new persona afterwards and emerged with one of the highest grade, but now I couldn't bear to look at the material without tearing up. It is all about how the neurons are connected and strengthened over time and if you could just connect it some how with the amygdala then you are set for a very long time.
    – Fraïssé
    May 3, 2015 at 23:40

There are many ways to reinforce what you have studied. Apply knowledge in different fields,like a person having studied JAVA can use that knowledge for developing softwares based on JAVA and softwares can be for Human Resource Management,Inventory Management,Data Mining etc. Teaching,helps to not only earn but also to find new avenues and new ways of expressing things. Revision at short intervals, this also reinforces learning. Sometimes people find their own unique of reinforcing knowledge like draw sketches,or use model language to prepare gist of what they have learned in such a manner that it takes only few hours to reinforce what they have learned over years. Software like Anki can also help if used regularly.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .