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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’:


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.


Real knowledge is to know the extent of one’s own ignorance. —Confucius

What if we are being radically deceived? What if I am no more than a brain kept alive in a vat and fed with stimuli by a mad scientist? What if my life is but a dream or computer simulation? Like the prisoners in Plato’s cave, I would be experiencing not reality itself, but a mere facsimile. I couldn’t be said to know anything at all, not even that I was being deceived. Given the choice between a life of limitless pleasure as a brain in a vat and a genuine human life along with all its pain and suffering, most people opt for the latter, suggesting that we value truth and authenticity, and, by extension, that we value knowledge for its own sake.

But even if we’re not being deceived, it is not at all clear that we can have any knowledge of the world. Much of our everyday knowledge comes from the use of our senses, especially sight. ‘Seeing is believing’, as the saying goes. French is one of many languages that has two verbs for ‘to know’: savoir and connaître, where connaître implies a kind of direct, privileged kind of knowledge acquired through sense experience. But appearances, as we all know, can be deceptive: a stick held under water appears to bend, the hot tarmac in the distance appears like a sparkling lake, and almost 40% of the normal population have experienced hallucinations of some kind, such as hearing voices. Our sense impressions are also subject to manipulation, as, for example, when a garden designer uses focal points to create an illusion of space. My mind interprets a certain wavelength as the colour red, but another animal or even another person may interpret it as something entirely else. How do I know that what I experience as pain is also what you experience as pain? You may react as I do, but that need not mean that you are minded like I am, or even that you are minded at all. All I might know is how the world appears to me, not how the world actually is.

Beyond my immediate environment, much of what I count as knowledge is so-called testimonial knowledge, that is, knowledge gained by the say-so of others, often teachers, journalists, and writers. If a piece of testimonial knowledge conflicts with our worldview, we tend, in the absence of non-testimonial evidence, to check it against other forms of testimony. If a friend tells me that Melbourne is the most populous city in Australia, I might carry out an Internet search and find that it is actually Sydney, even though I have never been to Australia and cannot be sure of what I read on the Internet.

Knowing that Sydney is the most populous city in Australia is a case of declarative (or propositional) knowledge, knowledge that can be expressed in declarative sentences or propositions. I know, or think that I know, that ‘Prince Harry is married to Meghan Markle’, ‘Paris is the capital of France’, and ‘democracy is the least worst form of government’. Apart from declarative knowledge, I also have know-how, for example, I know how to cook and how to drive a car. The relationship between knowing that and knowing how is not entirely clear, though it may be that knowing how collapses into multiple instances of knowing that.

For me to know something, say, that Mount Athos is in Greece, it must be the case that (1) I believe that Mount Athos is in Greece, and (2) Mount Athos is actually in Greece. In short, knowledge is true belief. True beliefs are better than false beliefs because they are, in general, more useful. Some beliefs, such as that my wine has been poisoned, are more useful than others, such as that my neighbour has 423 stamps in her collection. Some true beliefs, such as that I am a coward, can even be unhelpful, and we deploy a number of psychological mechanisms such as repression and rationalization to keep them out of mind. Inversely, some false beliefs, such as that my country or football team is the best, can be helpful, at least for my mental health. But on the whole we should seek to maximize our true beliefs, especially our useful or otherwise valuable true beliefs, while minimizing our false beliefs.

If knowledge is true belief, it is not any kind of true belief. People with paranoid psychosis often believe that they are being persecuted, for example, that the government is trying to have them killed. Clearly, this cannot count as knowledge, even if, by coincidence, it happens to be true. More generally, beliefs that are held on inadequate grounds, but by luck happen to be true, fall short of knowledge. In the Meno, Plato compares these true beliefs, or ‘correct opinions’, to the statues of Daedalus, which run away unless they can be tied down ‘with an account of the reason why’, whereupon they become knowledge. Knowledge, therefore, is not mere true belief, but justified true belief. Knowledge as justified true belief is called the tripartite, or three-part, theory of knowledge. Setting aside any intrinsic value that it may have, knowledge is more useful than mere true belief because it is more stable, more reliable.

Fine, but what does justification demand? I justify my belief in manmade global warming by the current scientific consensus as reported by the press. But what justifies my belief in the current scientific consensus, or in the press reports that I have read? Justification seems to involve an infinite regress, such that our ‘justified’ true beliefs have no solid foundation to rest upon. It may be that some of our beliefs rest upon certain self-justifying foundational beliefs such as the famous I think therefore I am of Descartes. But few beliefs are of this kind, and those that are seem unrelated to the bulk of my beliefs. In practice, most of our beliefs seem to rest upon a circular or circuitous chain of justification, which, if large enough, might be held to constitute adequate justification. The problem, though, is that people can choose to live in different circles.


People typically justify, or try to impose, their beliefs by means of arguments. Arguments provide reasons (or premises) in support of a particular claim or conclusion. There are two broad kinds of argument, deductive and inductive. In a deductive or ‘truth-preserving’ argument, the conclusion follows from the premises as their logical consequence. In an inductive argument, the conclusion is merely supported or suggested by the premises. More often than not, arguments are implicit, meaning that their rational structures are not immediately apparent and need to be made explicit by analysis.

A deductive argument is valid if the conclusion flows from the premises, regardless of the truth or falsity of the premises.

All organisms with wings can fly. (Premise 1, False)

Penguins have wings. (Premise 2, True)

Therefore, penguins can fly. (Conclusion, False)

This deductive argument is valid, even if it is unsound. For a deductive argument to be both valid and sound, all of its premises have to be true.

All mammals are warm-blooded. (Premise 1, True)

Bats are mammals. (Premise 2, True)

Therefore, bats are warm-blooded. (Conclusion, True)

Though a deductive argument appears to bring out a truth, that truth was already contained in the premises. For an inductive argument, the equivalent of soundness is cogency. An inductive argument is cogent if its premises are true and they render the truth of the conclusion probable. Every flamingo that I’ve ever seen has been pink. Therefore, it’s very probable that all flamingos are pink, or that flamingos are generally pink.

A third form of reasoning, abductive reasoning, involves inference to the best explanation for an observation or set of observations, for example, diagnosing a disease from a constellation of symptoms. But once broken down, abductive reasoning can be understood as a shorthand form of inductive reasoning.

Obviously, arguments often fall short. A logical fallacy is some kind of defect in an argument, and may be unintentional or intentional (with the aim to deceive). A formal fallacy is a deductive argument with an invalid form: the argument is invalid regardless of the truth of its premises. An informal fallacy is an argument that can only be identified by an analysis of the content of the argument. Informal fallacies are frequently found in inductive arguments, and often turn on the misuse of language, for example, using an ambiguous word with one meaning in one part of the argument and another in another (fallacy of equivocation). Informal fallacies can also distract from the weakness of the argument, or appeal to the emotions rather than to reason: “Will someone please think of the children!”

Science principally proceeds by induction, through the study of large and representative samples. An important problem with inductive reasoning is that the observations involved do not in themselves establish its validity, except by induction! A turkey that is fed every morning without fail expects to be fed every morning, until the day the farmer wrings its neck. For this reason, induction has been called ‘the glory of science and the scandal of philosophy’. This is an even bigger problem than it seems, since inductive arguments usually supply the premises for deductive arguments, which, as we have seen, are merely a priori. The 20th century philosopher Karl Popper argued that science actually proceeds by deduction, by making bold generalizations and then seeking to falsify them (or prove them wrong). He famously argued that if a proposition cannot be falsified, then it is not in the realm of science. But if Popper is right, then science could never tell us what is, but only what is not.

As we have seen, justification is hard to come by. But there is another problem lurking in the tripartite theory of knowledge. In 1963, Edmund Gettier published a two-and-a-half page paper showing that it is possible to hold a justified true belief without this amounting to knowledge. Here is my own example of a Gettier-like case. Suppose I am sleeping in my bed one night. Suddenly, I hear someone trying to unlock the front door. I call the police to share my belief that I am about to be burgled. One minute later, the police arrive and apprehend a burglar at my door. But it was not the burglar who made the noise: it was a drunken student who, coming home from a party, mistook my house for his own. While my belief was both true and justified, I did not, properly speaking, have knowledge. Responses to the Gettier problem typically involve elaborating upon the tripartite theory, for example, stipulating that luck or false evidence should not be involved. But these elaborations seem to place the bar for knowledge far too high.

As Gettier made clear, it is not so easy to identify instances of knowledge. Instead of defining the criteria for knowledge and, from these criteria, identifying instances of knowledge, it might be easier to work the other way, that is, begin by identifying instances of knowledge and, from these instances, derive the criteria for knowledge. But how can we identify instances of knowledge without having first defined the criteria for knowledge? And how can we define the criteria for knowledge without having first identified instances of knowledge? This Catch-22, in one form or another, seems to lie at the bottom of the problem of knowledge.