Most people are able to make the distinction between things that exist in the real world and things that are only imagined or are due to errors of perception. These kinds of errors can be visual, auditory, tactile, olfactory, or gustatory. Besides these, however, there is another common sort of perceptual error that is less often recognized as being purely subjective.
The purpose of our human perceptual system is to make sense of the world we live in, which it does by categorizing, into recognizable, pre-existing categories, the enormous amount of sensory data which our senses constantly receive and transmit to our brains. Without this continual process of categorization, this steady stream of data, as well as the world itself, would be completely meaningless and unrecognizable to us. There are, for example, some people who, because of a brain injury, are no longer able to carry out this usually automatic and involuntary process of categorization and recognition.
However, there is an unintended effect of this innate practical tendency to categorize things, which is that we sometimes see, hear, or otherwise perceive patterns that do not actually exist. A common example of this phenomenon is when we discern recognizable figures in stellar constellations or cloud formations. In the latter case, we all know that the figures we make out in these wispy, white, watery, whimsical, and wandering forms do not actually exist, because they are merely private projections onto the external reality. However, there are other situations where we fail to make this distinction, when we mistake the misleading effects of our perceptual system for something that exists independently of us.
There exist perceptual categories like tiger, snake, water, glass, plant, human, tornado, cliff, microbe, and life that are non-problematic, and whose existence, nature, and reality no one would dispute, except perhaps some philosophers. The important point to remember about our perceptual system, as well as the way we categorize the things that exist in the world, is that it evolved in order to help us to survive and procreate. But clearly, at least in the case of humans, perception can be, and often has been, used for other purposes, including creating and enjoying art, and devising theories to explain some of the features of the world we live in; or it produces results that have little or nothing to do with our ability to survive and procreate, such as leading people to believe in, worship, revere, and fear non-material entities such as gods, spirits, demons, karma, invisible energies, divine punishment or retribution, and other substances, forces, and occurrences.
Part of learning any new theory involves acquiring its particular definitions, or categories, into which the events and phenomena of the world are classified. For instance, the communist categorizes society and historical events into categories such as bourgeois, capitalist, exploited worker, proletariat, class struggle, workers’ revolution, and so on. The scientist categorizes the world into electrons, protons, neutrons, atoms, molecules, gravity, elements, electromagnetic force, cells, genes, and species, none of which categories existed before the scientist created or discovered them. The psychoanalyst categorizes human phenomena into the categories of conscious or unconscious desire, repressed desire or feeling, libido, id, ego, superego, sublimation, the Oedipus or Electra Complex, death wish, castration fear, and penis envy. The behaviourist categorizes human phenomena into the categories conditioned reflex, positively or negatively enforced behaviour, stimulus, response, reward, punishment, and learned behaviour. The theory of imitation categorizes human behaviour into the categories norm, model of behaviour, multi-person model, situational behaviour, realm of influence, imitation, conformity, admiration, contempt, and embarrassment.
What these and other examples show is that there are many different ways of categorizing the same phenomena. This is especially true of human behaviour, about which there has been proposed a multitude of different theories in order to account for it. But clearly not all of these different ways of categorizing the world, or a certain part of it, are equally valid or useful. Just as we can see recognizable forms in wispy cloud formations, which forms are soon obliterated, and just as different people sometimes see different forms in the same cloud formations, it is important for people to realize that it is quite easy for us to be misled by seeing patterns, regularities, or structures in the world around us which don’t actually exist, or have only a personal significance. This becomes more likely when we experience a strong feeling, such as love, anger, hatred, admiration, contempt, embarrassment, excitement, fear, enthusiasm, greed, lust, disgust, or boredom, all of which can strongly colour or influence the way we perceive a person, object, place, or event.
There is an area of our lives in which it is very easy to see or project unrealistic or irrational expectations, and that is the future. Because the future has not yet occurred, many of us hold false beliefs about what will occur, such as in regards to ourselves. I suppose that most of us have had the experience of believing that someone we are enamoured of is favourably disposed towards us. We fit the facts of the person’s behaviour into a pattern of acquaintance, interest, attraction, mutual passion, marriage or relationship, and so on. But it is not only in matters of love that our human tendency to see patterns where none exist can mislead us. The inventor sees oneself becoming wealthy and renowned due to one’s brilliant original invention; the athlete sees oneself winning a gold medal or championship in one’s sport; the scientist imagines one’s research rewarded with a Nobel Prize; the actor or musician hears the ecstatic applause of the adoring crowd; the parent sees one’s child becoming successful or famous in some field of endeavour; and the author sees oneself becoming celebrated for one’s latest book, which is read admiringly by millions of people worldwide. Many of these visions of what we believe will happen in the future are just as ephemeral and unreal as the shapes that we see in clouds. Even in more mundane matters, such as buying a house, many people see patterns based solely on what has happened in the recent past, such as constantly rising real estate prices. In order to pay the mortgage on the house, most people assume that they will continue to earn money by working, or that, in the worst case scenario, they can resell the house at the same or a higher price, which, however, does not always turn out to be true.
There are many gamblers and financial speculators who see or look for such patterns in the phenomena they observe or study, and consequently are misled by believing that there exist regularly occurring patterns in phenomena that are, for the most part, completely random. For instance, a sequence of unlikely events, such as a continual succession of heads in a coin toss, double sixes in dice, a certain number or colour in roulette, or a constantly rising stock or real estate market, leads many people to believe that there exists a discernible pattern, and therefore the next occurrence will continue the trend, even though the laws of probability tell us otherwise. This is more likely when they experience a strong feeling of conviction that their belief is correct. Another example of this common error is when a person prays to God, and the things one asks God for come to pass, one then concludes that one is favoured or blessed by God. This fortunate person fails to consider that there are a great many people – literally billions and billions of them – who pray to God, and so it will inevitably happen that some of them will seem to have their wishes fulfilled and their prayers answered by divine favour or intervention.
Advertisers often use this innate human tendency to get people to buy their products. Some examples are showing only professional athletes who use a product and are muscular, physically fit, and successful, or presenting only formerly obese persons who have lost a lot of weight by following a diet or using a weight-loss product, or a mutual-fund company that presents only the large gains that it made during a string of successful years, which leads the gullible consumer to believe that a pattern exists, and that this pattern will continue in the future.
The lesson to be drawn from the many false beliefs and theories which, during some period of history, have been accepted as true by a large number of people, is how widespread and persistent these errors or illusions of categorization can be. Even in the present day, there are many people who believe in astrology, numerology or other numerical patterns, communism, psychoanalysis, and free-market capitalism, all of which are false theories about human behaviour that have deceived many millions of people with their seductive but erroneous claims about human nature and behaviour. And this is because our human brain has an innate tendency to try and impose some kind of coherent order on the phenomena which it observes, regardless of whether this order or regularity actually exists in the world. Considering how much harm has resulted from such errors of perception, we would do well to be constantly on our guard against them, and not believe what some admired person or widely accepted theory tells us about the patterns that are discernible in the phenomena of the world we live in.
 Of course, it is possible for people to acquire new perceptual categories or distinctions, such as a person who studies a certain branch of art like Gothic architecture and learns to distinguish it from other architectural styles, while being able to recognize its characteristic features in buildings that one hasn’t seen before.
 See, for example, Oliver Sacks’ fascinating account of such a person in The Man Who Mistook His Wife for a Hat.
 I am fully aware that the use of the words “internal” and “external” in this context are philosophically highly problematic, especially considering that I have declared elsewhere that everything that we see, hear, and perceive in other ways is created by our brains. But since I regard philosophers as abject fools, and many of the things they are wont to spend their time doing completely frivolous, I will not consider this matter in any detail, although others may of course do so.
 In the realm of art, literature, and music, these kinds of subjective interpretations abound. There are some academics, critics, and professors who heatedly argue with each other about whose interpretation of a well-known artwork is correct. These disputants would do well to remember that many of these academic disputes are much like debates about the forms that different people see in cloud formations, and are about as significant, or insignificant.