 Cultural units are not replicators: Replicators, remember, are supposed to be units that make copies of themselves. Some critics of the meme concept argue that there is no known mechanism that could explain how memes are copied. But this is a mistake. An idea can be copied simply through observation and inference: agent B can observe the behaviour of agent A, infer that A holds some belief X, and thereby come to hold the same belief as A. Ideas can also be copied using linguistic communication. Agent A might be convinced of belief X, she tells B about it, and B comes to believe X too. In both cases one can say that belief X makes a copy of itself, albeit via language, inference, and so forth. A more pressing worry for memetics is that imitation is usually too error-prone to underpin replication. If I make a cake on the basis of a secret family recipe, you eat the cake, and you then attempt to make another one, then the chances are that the recipe you hit upon will not, in fact, be exactly the same as the one I used, even if you are able to make a similar-tasting cake. Another significant worry for memetics is that when the same ideas do spread through a population, it is rarely because they are literally copied from each other. Sperber argues that cultural reproduction is rarely meme-like, but instead makes use of what he calls ‘attractors’—culturally shared patterns of thought, which enable representations to spread through a population without literal copying. Sticking with the cake example, perhaps you eat a slice of my Victoria sponge, you like it, and you decide to make one for yourself. Perhaps the recipe you use is very similar to mine. But you have not figured out by tasting my cake which ingredients needed to go in and in what order. Rather, you already knew how to make a Victoria sponge. Eating my cake simply triggered the use of a recipe that was already in your repertoire. In this case, the recipe has appeared in my head, and because of this it has appeared in yours, but not because your recipe is a copy of mine. Sperber argues that memetics is mistaken because most cases of the spread of ideas are like this: