Memes, Dreams, and Themes
This is to be an exercise in the taxonomy of ideas. It is characteristic of ideas to be shared by many minds. Why is this? One reason is that ideas spread from mind to mind. Here is where the concept of a meme comes in: a meme spreads like a virus from one mind to another, duplicating itself, colonizing new minds. Memes include jingles, catchphrases, fads, fashions, crazes, religions, ideologies, mannerisms, and accents. They spread by imitation and credulity, exploiting the receptivity of the human mind to new information and influence. They may mutate and be subject to natural selection, sometimes proliferating wildly, before possibly going extinct. Thus ideas (in a broad sense) exist in many minds because they are memes: they have arrived there from somewhere else by means of meme transfer. The mechanism of meme transmission is essentially mental manipulation: minds are disposed to accept whatever comes along, as a result of the childhood need to absorb as much information as possible in the shortest time, and so uncritical copying is favored. People just can’t help picking stuff up, willy-nilly. Memes are like computer viruses—they trade on the architecture of the system to insert themselves into the software. Once inside they can vary from mild mental nuisance to dangerous ideology. In some respects they work like a drug: they trigger reactions in our brains that take over our minds. That annoying jingle in your head is a meme playing with your brain chemistry. 
The concept of the meme can be taken more or less widely. Some people take it to provide a general theory of human culture and idea transmission. I want to distinguish the meme from two other sorts of idea that are importantly different from it; thus I am providing a taxonomic survey of ideational contents. My taxonomy invokes a three-way division: memes, dreams, and themes. First I consider dreams. Dream ideas are also widely shared, with the same kinds of dream cropping up in different communities and cultures. Moreover, like memes, these dream contents often seem rather arbitrary and pointless—despite being widely shared. Thus people regularly dream of falling, flying, being pursued, being embarrassed, missing trains or buses, being inadequately prepared, being incapacitated, and finding an extra room in the house. The last item is particularly peculiar: why should so many people dream of that? It is fair to say that no one knows why people dream as they do, though theories abound. But one thing is clear: it is not by means of imitation. It is not that dreamers transmit their dreams to others by recounting them or otherwise making them public (say, by making a film embodying the dream). People just tend spontaneously to have the same sorts of dream. So dream ideas are not shared because they are transmitted like memes: they don’t spread like a virus from one mind to another; they are not the result of copying. Possibly dream life can be influenced to some degree by shared culture in meme-like fashion, but that is not the explanation of the majority of common dream content. Dreams seem to grow from within, like bits of anatomy; they are not picked up from interactions with others. Memes are exogenously formed; dreams are endogenously formed. So dreams are not memes.
My third category I call themes, partly for the sake of the rhyme, but also because it has a breadth that I want to emphasize. One of the salient features of memes is that they do not spread by rational persuasion—they spread by non-rational manipulation. But the spread of scientific ideas, to take the most obvious example, is not like that (though there are theorists would like to extend the meme concept to scientific belief): scientific ideas spread because they have been found to be true—or at least have been empirically confirmed. Scientific beliefs are shared because of the existence of publicly available rational justification. Thus Darwinism, for example, is generally accepted because of the overwhelming evidence in its support, and similarly for other accepted scientific theories. There is no exploitation of weakness in the scientist’s mind, no drug-like manipulation, no lethal catchiness. The explanation for the spread of scientific ideas is simply the power of scientific method and rational persuasion. On no account should we assimilate the transmission of scientific knowledge to meme transmission; indeed, the concept of the meme is intended precisely to mark that contrast. Memes spread for reasons that are independent of rationality, not by virtue of rationality.
I hope what I have just said is completely uncontroversial, because now I want to court controversy. It would surely be wrong to restrict the non-meme type of idea transmission to science: many other disciplines involve shared beliefs, where these beliefs are shared for good rational reasons. Thus: history, geography, literature, philosophy, mathematics, music theory, engineering, cookery, and bottle washing. There is a large range of human cognitive activities in which ideas are shared by something other than meme propagation—not all of them counting as “science”. We clearly need to expand the notion of rationality so as to incorporate these areas. And there is no difficulty in doing so: there are standards of evidence, argument, and intellectual rigor that characterize all these areas—it isn’t all jingles and ideology (despite what “post-modernists” may claim).
But matters get a bit more interesting when it comes to aesthetics. Aesthetic ideas spread—is this kind of spread more like meme transmission or scientific communication? Compare an advertising jingle to the opening bars of Beethoven’s Fifth Symphony. Both may lodge in the mind against one’s will, repeating themselves endlessly; and they may be transmitted to others, say by whistling. Are they both therefore memes? I would say not. There is a different explanation of the musical spread in each case: in the jingle case we have a meme, a worthless cultural trope that insidiously takes over the mind; but in the Beethoven’s Fifth case we have an aesthetically valuable musical theme. And that is why I call my third category “themes”: themes are cultural units with intrinsic value, which deserve to be spread and replicated. They are not annoying mind viruses but welcome friends, sources of personal enrichment. The reason they spread is that they are excellent—meritorious, worthwhile—and are recognized to be so. They are not science but they are not mere memes either. 
Notice that we can only distinguish themes from memes by employing evaluative language, and by assuming that values play a role in cultural transmission. Themes spread because they have value, while memes spread despite having no value. It is the same with other aesthetic products—such as art and literature. Famous lines from Shakespeare don’t spread because they are memes—worthless cultural viruses—but because they are judged to be aesthetically valuable, and rightly so judged. It is a matter of merit. This is not a high culture versus low culture point: a good Beatles song, say, is a completely different animal from a commercial jingle. The point is that the mechanism of transmission is quite different in the two cases, being more like science in the theme case, in contrast to your typical meme. Whereas we can say that we are suckers for memes, we cannot say we are suckers for themes.
None of this is to deny that memes and themes can get mixed up in practice, or that it is always easy to tell the difference. There can be fads and fashions in science too—memes masquerading as themes (the idea of a “paradigm shift” comes to mind). But there is a deep difference of principle here— there are two very different kinds of idea transmission. Memes may even disguise themselves as themes in order to gain a stronger hold, as with certain “scientific” ideologies. The difference lies in the psychological means of transmission. Themes may spread from mind to mind in an epidemiological manner, even mutating as they spread, but the reason for their exponential spread is not the same as for the case of memes. In the latter case it is brute susceptibility, but in the former case it is appreciation of merit. This is why we don’t resent the transmission of themes into our minds, while we do resent the insertion of memes (at least those recognized to be such). Theme transmission is genuine learning or improvement, but meme transmission involves no learning or improvement, merely mental infection. One of the central questions of cultural life is which of one’s existing ideas are memes and which are themes: which are absorbed because of mental manipulation and which have genuine cognitive value? That important question is possible only if we decline to extend the concept of a meme beyond its legitimate domain.
In sum: there are three categories of ideas in the mind, differing in their etiology; none can be assimilated to the others. Memes are ideas that spread by non-rational means, bypassing our critical faculties. Dreams incorporate ideas that don’t spread at all, or very little, apparently arising from within. Themes are ideas that spread and proliferate, but they do so by appealing to our rational and critical faculties. Memes expose our weakness; themes demonstrate our strength; dreams reveal our oddity. 
 In ethics we also find a meme-theme distinction: some ethical ideas spread by manipulative propaganda, while others spread by virtue of their inherent cogency (I leave it to the reader to supply her own examples). Politics may be viewed as the battle between ethical memes and ethical themes—propaganda versus merit.
 I don’t say anything here about the contribution of genes to the distribution of ideas across minds. But for anyone who believes in innate ideas, genes are the basis for some of our ideas; so it is genes that explain why ideas of this class are shared—we share genes for the same set of innate ideas.