Memory refers to the system, or systems, by which the mind registers, stores, and retrieves information for the purpose of optimizing future action.

Memory can be divided into short-term and long-term memory, and long-term memory can be further divided into episodic and semantic memory. Episodic memory records sense experiences, while semantic memory records abstract facts and concepts, with episodic memories eventually seeping into semantic memory. Interestingly, the distinction between episodic memory and semantic memory is already implicit in a number of languages in which the verb ‘to know’ has two forms, for example, in French, connaître and savoir, where connaître implies a direct, privileged kind of knowledge acquired through sense experience.

There is, naturally, a close connection between memory and knowledge. The connaître and savoir dichotomy is also pertinent to the theory of knowledge, which distinguishes between first-hand knowledge and testimonial knowledge, that is, knowledge gained through the say-so of others, often teachers, journalists, and writers. In the absence of first-hand knowledge, the accuracy of a piece of testimony can only be verified against other sources of testimony. Similarly, the accuracy of most memories can only be verified against other memories. For most if not all memories, there is no independent standard.

Episodic and semantic memory are held to be explicit or ‘declarative’, but there is also a third kind of memory, procedural memory, which is implicit or unconscious, for knowing how to do things such as reading and cycling. Although held to be explicit, episodic and semantic memory can influence action without any need for conscious retrieval—which is, of course, the basis of practices such as advertising and brainwashing. In fact, it is probably fair to say that most of our memories lie beyond conscious retrieval, or are not consciously retrieved—and therefore that memory mostly operates at an unconscious level. ‘Education’, said BF Skinner, ‘is what survives when what has been learnt has been forgotten.’

A mysterious type of memory is prospective memory, or ‘remembering to remember’. To send my mother a birthday card, I must not only remember her birthday, but also remember to remember it. Whenever I omit to set my alarm clock, I find myself waking up just in time to make my first appointment, even when I have only slept three or four hours. This suggests that, even in sleep, the mind is able to remember to remember, while also keeping track of the time.

Memory is encoded across several brain areas, meaning that brain damage or disease can affect one type of memory more than others. For example, Korsakov syndrome, which results from severe thiamine deficiency and consequent damage to the mammillary bodies and dorsomedial nucleus of the thalamus, affects episodic memory more than semantic memory, and anterograde memory (ability to form new memories) more than retrograde memory (store of old memories), while sparing short-term and procedural memory. Alzheimer’s disease on the other hand affects short-term memory more than long-term memory, at least in its early stages.

As a psychiatrist, I am often asked to assess people with advanced Alzheimer’s Disease and other forms of dementia, and am all too aware of the importance of memory in our daily lives. Without any memory at all, it would be impossible to: speak, read, learn, find one’s way, make decisions, identify or use objects, cook, wash, dress, or develop and maintain relationships. To live without memory is to live in a perpetual present, without past, and without future. One couldn’t build upon anything, or even engage in any kind of sustained, goal-directed activity. Although there is wisdom in being in the moment, one cannot always or entirely be in the moment. In Greek myth, the goddess of memory, Memosyne, slept with Zeus for nine consecutive nights, thereby begetting the nine Muses. Without memory, there would be no art or science, no craft or culture.

And no meaning either. Nostalgia, sentimentality for the past, is often prompted by feelings of loneliness, disconnectedness, or meaninglessness. Revisiting our past can lend us much-needed context, perspective, and direction, reminding and reassuring us that our life is not as banal as it might seem, that it is rooted in a narrative, and that there have been—and will once again be—meaningful moments and memories. If weddings and wedding photographs are anything to go by, it seems that we go to considerable lengths to manufacture these anchor memories. Tragically, people with severe memory loss can no longer revisit the past, and may resort to confabulation (the making up of memories) to create the meaning and identity that everyone yearns for. I once visited a nursing home in England to assess an 85-year-old lady with advanced Alzheimer’s disease. She insisted that we were in a hotel in Marbella: she was planning her wedding and didn’t have time to talk to me. When I asked her what she did the day before, she replied, with a twinkle in her eye, that she hit the town for her bachelorette (hen night), and that her glamorous friends spoilt her rotten with champagne and fancy cocktails. The search for meaning is deeply ingrained in human nature, so much so that, when pressed to define man, Plato replied simply, ‘a being in search of meaning’.

It could be argued that, like confabulation, nostalgia is a form of self-deception, in that it involves distortion and idealization of the past. The Romans had a tag for the phenomenon that psychologists have come to call ‘rosy retrospection’: memoria praeteritorum bonorum, ‘the past is always well remembered.’ And memory is unreliable in other ways as well. ‘Everyone’, said John Barth, ‘is necessarily the hero of his own life story’. We curate our memories by consolidating those that confirm or conform to our idea of self, while discarding or distorting those that conflict with it. We are very likely to remember events of existential significance such as our first kiss, or our first day at school—and, of course, it helps that we often rehearse those memories. Even then, we remember just one or two scenes, and just the main features, and fill in the gaps and background with reconstructed or ‘averaged’ memories. Déjà-vu, the feeling that a situation that is currently being experienced has already been experienced, may arise from a near match between the current situation and an averaged memory of that sort of situation. Our memories are filtered through our interests and emotions, and through our interpretation of events. Two people supporting opposing teams in a football match, or opposing political parties in an election, will register and recall very different things, and would likely disagree about ‘the facts’.

Broadly speaking, emotionally charged events are more likely to be remembered, and it has been found that injections of cortisol or epinephrine (adrenaline) can improve retention rates. But if a situation is highly stressful, memory may be impaired as cognitive resources are diverted to dealing with the situation, for example, escaping from the gunman rather than registering his clothing or facial features. In addition, any attention paid to the gunman is likely to focus on the gun itself, leading to a kind of peripheral blindness. This has important implications for the accuracy of eyewitness testimony, which might also be distorted by the use of leading or loaded questions. In a famous study, Reconstruction of Automobile Destruction, Loftus and Palmer asked people to estimate the speed of motor vehicles when they smashed / collided / bumped / hit / contacted each other, and found that the verb used in the question altered perceptions of speed. In addition, those asked the ‘smashed’ question were subsequently more likely to report having seen broken glass. After a traumatic event, to cope with unbearable stress, a person might go so far as to dissociate from the event, for example, by losing all memory for the event (dissociative amnesia) or even, as Agatha Christie once did, assuming another identity and departing on a sudden, unexpected journey (dissociative fugue). So emotion improves memory, but stress and trauma hinder it.

It is generally thought that, of all the senses, it is the sense of smell that triggers the most vivid memories. The olfactory bulb has direct connections to the amygdala and hippocampus, which are heavily involved in memory and emotion. These three structures—the olfactory bulb, the amygdala, and the hippocampus—form part of the limbic system, a ring of phylogenetically primitive, ‘paleomammalian’ cortex that is the seat of memory, emotion, and motivation. In a famous passage now referred to as ‘the madeleine moment’, Marcel Proust describes the uncanny ability of certain smells to recapture the ‘essence of the past’:

No sooner had the warm liquid mixed with the crumbs touched my palate than a shudder ran through me and I stopped, intent upon the extraordinary thing that was happening to me. An exquisite pleasure had invaded my senses, something isolated, detached, with no suggestion of its origin. And at once the vicissitudes of life had become indifferent to me, its disasters innocuous, its brevity illusory – this new sensation having had on me the effect which love has of filling me with a precious essence; or rather this essence was not in me it was me. … Whence did it come? What did it mean? How could I seize and apprehend it? … And suddenly the memory revealed itself. The taste was that of the little piece of madeleine which on Sunday mornings at Combray (because on those mornings I did not go out before mass), when I went to say good morning to her in her bedroom, my aunt Léonie used to give me, dipping it first in her own cup of tea or tisane. The sight of the little madeleine had recalled nothing to my mind before I tasted it. And all from my cup of tea.

Killing two birds with one stone, here are 10 ways to improve your memory that also shed light on its workings:

1. Get enough sleep. If you read a book or article when very tired, you will forget most of what you have read. Sleep improves attention and concentration, and therefore the registration of information. And sleep is also required for memory consolidation.

2. Pay attention. You cannot take in information unless you are paying attention, and you cannot memorize information unless you are taking it in. It helps if you are actually interested in the material, so try to develop an interest in everything! As Einstein said, ‘There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.’

3. Involve as many senses as you can. For instance, if you are sitting in a lecture, jot down a few notes. If you are reading a chapter or article, read it aloud to yourself and inject some drama into your performance.

4. Structure information. If you need to remember a list of ingredients, think of them under the subheadings of starter, main, and dessert, and visualize the number of ingredients under each subheading. If you need to remember a telephone number, think of it in terms of the first five digits, the middle three digits, and the last three digits—or whatever works best.

5. Process information. If possible, summarize the material in your own words. Or reorganize it so that it is easier to learn. With more complex material, try to understand its meaning and significance. Shakespearean actors find it much easier to remember their lines if they can understand and feel them. Focus on the bigger picture, not the details, and don’t remind everyone of everything. In the words of Oscar Wilde, ‘One should absorb the colour of life, but one should never remember its details. Details are always vulgar.’

6. Relate information to what you already know. New information is much easier to remember if it can be contextualized. In a recent study looking at the role of high-level processes, Lane and Chang found that chess knowledge predicts chess memory (memory of the layout of a particular game of chess) even after controlling for chess experience.

7. Use mnemonics. Tie information to visual images, sentences, and acronyms. For example, you might remember that your hairdresser is called Sharon by picturing a Rose of Sharon or a sharon fruit. Or you might remember the colours of the rainbow and their order by the sentence, ‘Richard Of York Gave Battle in Vain’. Many medical students remember the symptoms of varicose veins by the acronym ‘AEIOU’: Aching, Eczema, Itching, Oedema, and Ulceration.

8. Rehearse. Sleep on the information and review it the following day. Then review it at growing intervals until you feel comfortable with it. Memories fade if not rehearsed, or are overlain by other memories and can no longer be accessed.

9. Be aware of context. It is easier to retrieve a memory if you find yourself in a similar situation, or similar state of mind, to the one in which the memory was formed. People with low mood tend to remember their losses and failures while overlooking their strengths and achievements. If one day you pass the cheesemonger in the street, you may not, without her usual apron and array of cheeses, immediately recognize her as the cheesemonger, even though you know her fairly well. If you are preparing for an exam, try to recreate the conditions of the exam: for example, sit at a similar desk, at a similar time of day, and use ink on paper.

10. Be creative. Bizarre or unusual experiences, facts, and associations are much easier to remember. Because unfamiliar experiences stick in the mind, trips and holidays give the impression of ‘living’, and, therefore, of having lived a longer life. Our life is just as long or short as our remembering: as rich as our imagining, as vibrant as our feeling, and as profound as our thinking.