Why Do We Imagine?
If we ask why humans perceive and remember, the answer is not far to seek: for the same reason many animals, particularly mammals, perceive and remember, viz. these are obviously useful traits to possess. They enable the reception and storage of information. There is no evolutionary puzzle about the existence of perception and memory in humans (though aspects of these traits can be puzzling). The function of such traits is evident and indisputable (compare muscle and bone). But what about imagination? Imagination isn’t about the reception and storage of information, so it can seem frivolous and surplus to requirements; and it is not so commonly possessed as perception and memory. In fact, it is hard to know which animals possess it—there is no obvious behavioral sign of imaginative mental activity. Do cats and dogs have mental images? If so, can they control them as we can? Can a dog form the image of a horse with a human head? How would we find out? Can a gorilla imagine that he is floating down a river or is hairless? We don’t know. What does seem clear, however, is that human imagination is particularly vivid, extensive, and creative: we are imaginative creatures, endowed with powerful imaginations. The question is why: what led us to acquire such a faculty, and what function did it (and does it) perform? What kind of selective pressure worked on us to develop a brain that permits imaginative mental acts? What’s it for? Why are we so extravagantly imaginative?
We need something specific not just airy handwaving about the glories of the human mind. I think we already have the materials with which to answer the question: it is because we are explorers and travelers, geographically curious and peripatetic; and also, we are builders of artifacts, particularly homes. We are nomadic and industrious: we go on journeys and we make useful objects. To do these things, and to do them intentionally, we need to envisage and anticipate the intended goal: a new habitat, a new dwelling. And this involves the use of imagination—conjuring up in the mind’s eye what it is we seek to achieve. We dream of foreign lands and commodious residences, even if they are only on the other side of that mountain or a modest bamboo shack (we are to think of early humans with realistic goals). Animals that stay put and don’t build anything have no pressing need for an imagination, but once a creature sets out to take a trip or build a home it is necessary for it to picture the future. Expeditions and construction projects are exercises in future management, and imagination is the faculty by which the future is represented. Cats perceive the present and remember the past, but they don’t (as far as we know) imagine the future—they don’t plan, as humans do. We are future-oriented beings, and so we have a use for a faculty of imagination. Migratory and nest-building animals may also have a use for the imagination, envisaging (predicting, expecting) the results of their labors—birds, for example. Whales don’t build nests but they do take long trips, perhaps imagining their destination (all the tasty plankton they will eat). By the time humans mastered the maritime arts they were in full imaginative swing, picturing far-off exotic lands in specially constructed house-like boats. So, we get a neat explanation of imagination based on materials already employed in the explanation of spoken language and intelligent thought, which is a nice confirmation of the overall approach. Imagination enables us to migrate better and build better, so it has adaptive advantage. This is why we are so singularly imaginative, the imagination champions of the animal world—we simply have a greater use for the imagination. Possibly, construction-directed imagination came first, soon after our descent from the trees, to be followed by a re-deployment of the imagination in the search for new territories, and later to flower into the many realms in which imagination now flourishes. Imagination wasn’t just a gratuitous excrescence that added to our enjoyment of life (art, literature, the opera); it was a biological necessity, what stood between us and extinction. For we could not live without imagining future artifacts and hunting grounds—these mental acts enabled us to make the imagined things into realities. Imagination enabled us to defeat tigers and the cold (by building protective dwellings) and prey scarcity and rival tribes (by moving to new territories).
You must be wondering: what about sexual fantasy? What explains that? Good question—because what we have so far falls short of an adequate explanation of the human activity of sexual imagining. True, there is an element of planning and walking around, and so imagining the future; but other animals manage sex without a full-blooded sexual imagination. Nor is there much in the way of house building in sexual fantasy. And it has to be admitted that human sexual imagination is strikingly lively, forceful, time-consuming, and tenacious—why so animated, so gripping? Evidently, we need to introduce a further explanatory factor, over and above travel and carpentry work. The obvious thought is that sex needs to overcome some sort of barrier and sexual fantasy has been enlisted in that effort. Certainly, sexual desire is intimately linked to sexual fantasy, each driving the other—in particular, fantasy makes us want sex more. Without it wouldn’t sex be less compelling, more routine, not as…imaginative? But what kind of barrier might a lively sexual imagination operate to overcome? I can think of two possibilities: violence and disease. To engage in sex is to risk violence: the partners are physically vulnerable, especially the female; defloration can be painful and bloody; the act of intercourse can look and sound violent (I have read that shark sex actually is quite violent, the male shark biting off chunks of the female!). All this is enough to inhibit a sensible creature from engaging in sex, especially a female human; so, it might be necessary to subdue such fears with a compelling desire-infused sexual imagination. This suggestion has the obvious problem that it applies primarily to the female (though some mating female spiders are notoriously homicidal), but the male also has a strong sexual imagination. Thus, the disease explanation recommends itself: both partners could be equally afraid of disease and hence disinclined to engage in sexual intercourse. Recall the prevalence of syphilis in earlier ages (it was probably much worse in prehistoric times when the genetic blueprint was laid down). That would be the end of the line as far as the genes are concerned—no more intercourse, therefore no gene survival. True, the partners might not survive long after the sexual act if it involves the transmission of a deadly disease, or even a debilitating one; but not to have sexual intercourse at all is far worse as far as the genes are concerned. So, they need to install a trait that works against the fear of disease—they accordingly pump up the sexual imagination. They make it into a Siren song, an offer that can’t be refused, a hard Yes (so to speak). They make it scream, “Do it!” So, the biological explanation of the human sexual imagination is that it is designed as a counterweight to disease phobia (or rational disease avoidance). It blots out the prudential pleadings of the disease-avoidance instincts of the organism–brackets them, silences them. Other animals have no knowledge of STDs, so happily proceed to the main event in blissful ignorance (there is actually a lot of sexually borne disease in animal populations); they thus have no need of an irresistible sexual imagination that gets them over the hump. But humans are sexually inhibited in this respect, so the genes have resorted to the expedient of installing a gadget that de-activates the fear reflex—a red hot sex-picturing device. Sexual fantasy is thus explained as a mechanism of (understandable) fear management.
According to the preceding theory, human sexuality is the product of a war conducted at the level of the genes (among other things). We have a gene for shunning sex (I am oversimplifying) and we also have a gene for welcoming it (ditto). The human sexual imagination is a consequence of that war. This kind of conflict or ambivalence is not uncommon in genetic programming: we may be genetically disposed to eat mushrooms, but also wary of eating a strange-looking fungus; or a stag may wish to obtain mating privileges over another stag, but not relish butting heads with that stag. Animals want to keep safe (and their genes want that too) but they also want get ahead in life. Just so, humans want sex but they also want to avoid catching the diseases associated with it. The genes must balance risk from intercourse with the necessity of impregnation; according to the present theory, they designed the human sexual imagination so as to overcome the inhibitory effects of disease avoidance. Both of these imperatives are strong, because they bear directly on gene reproduction, and so sexual imagination is a site of warring emotions—fear and desire combined. By all means take a risk, the genes say, but don’t take too much of a risk. The sexual urge must be powerful but not overwhelming. That is the explanation of its distinctive phenomenology and causal profile. If we add it to the previous two factors—travel and homemaking—we get the evolutionary origins of human imagination in general. Speech, thought, and imagination cluster together around these basic facts of human biology. They reflect ground-floor biological imperatives (travel, build, reproduce).
 I hope readers have the good sense to take gene personification with a grain of salt: it is simply convenient shorthand, easily dispensable in favor of more cumbersome formulations. Also, of course, I am presenting theory-sketches here; much more work, conceptual and empirical, would need to be done.