The episode of the madeleine in the Proust work cited above is one of the most famous passages of universal literature of all times. Not only is it beautifully written, but the passage also describes an experience that is so personal and so ubiquitous in human nature that any psychologist, from Freud to Pavlov, would love to explain it. We will refer to it as the “Proust effect.” To our knowledge, it is the best possible description of associative learning.

The beauty of the target article by Mitchell et al. is that it tries to understand the Proust effect in its entirety, not just a part of it. As such, the article is ambitious, important, and timely. It makes us rethink all the established assumptions about learning. Contrary to all intuitions, Mitchell et al. (almost) convince us that (a) there must be only one learning process, and (b) this unique process must be propositional in nature.

The standard explanation for associative learning is the link approach. Because the narrator in Proust's novel had associated the madeleines with all the happiness of childhood (even though he was not aware of this fact), then tasting one of those cakes now, after so many years, brought back the enormous happiness and all the good feelings from childhood. Thus, the Proust effect reflects a simple, automatic link that was created during childhood and is now expressed, also without effort or knowledge of the contingencies, in the form of a conditioned response (CR). According to the link proponents, there was no propositional learning here, no consciousness of the contingencies while the association was acquired; not even now that it is expressed. Indeed, it will still take the narrator many pages and a considerable amount of thinking and elaborated reasoning to discover why the madeleine was producing the CR.

But the link approach is not as simple as it seems, and Mitchell et al. are correct in highlighting this point: The link approach presupposes a dual (and complex) system. Automatic links need to be complemented with some more-elaborated, rational, and time-consuming forms of learning. This complex learning is at work, for instance, after the CR has occurred and the narrator begins to consciously think about it and tries to identify its cause. Even the most enthusiastic proponents of low-level mechanisms have to admit that people are obviously capable of other forms of learning and reasoning.

What Mitchell et al. suggest is that, if we all agree that propositional learning is needed, why should we maintain a belief in automatic links? Couldn't we assume just a propositional learning process that could account for both the automatic-like and the more complex processes? Are there any experiments that can only be explained by the link mechanism? That there are data to support that propositional learning exists is unquestionable, and the authors make an excellent case of it. That many of the results that have traditionally been explained using the link approach can also be explained by the propositional account is also clear in their target article. Moreover, it is well established today that there are very few experiments that can be explained solely by the link approach (Lovibond & Shanks Reference Lovibond and Shanks2002; Shanks & St. John Reference Shanks and St. John1994). What Mitchell et al. are showing is that both the dual and the propositional account can explain the majority of the available evidence. Scientific parsimony becomes then the central argument: If a single process can explain it all, why should science maintain two?

But the argument of scientific parsimony should be confronted against that of natural selection. A simple, low-level process is vital for survival because, by definition, it can do all those things the complex process cannot do: it responds quickly, automatically, and without consciousness or effort to the demands of the environment. Even under high pressure, it provides a fast tool for survival. Its loss would be too costly.

As Mitchell et al. note, natural selection has produced a continuum of complexity in the different species. At one end of this continuum, we find very simple species which have just the link system and no cognition. At the other end, we find the human species, which, according to Mitchell et al., has only the propositional system. If so, Mitchell et al. need to explain why humans (and other evolved animals) should have lost their primitive link system while developing the propositional one. There is no clear evolutionary advantage in losing a fast and automatic tool.

Indeed, there is a growing body of evidence suggesting that learning is actually caused by a multiplicity of different mechanisms and that the insistence of traditional learning theory in a unique, general-purpose learning system was simply a mistake (Gallistel Reference Gallistel and Gazzaniga2000; Tooby & Cosmides Reference Tooby, Cosmides, Barkow, Cosmides and Tooby1992; Reference Tooby, Cosmides and Buss2005). If natural selection has encouraged flexibility and adaptability, having many different forms of learning must have been favored through the course of evolution.

In sum, Mitchell et al. need to explain not only why consciousness becomes so difficult in the Proust effect, but also what survival advantages a species that extinguishes the link system should have. If all the evidence for the automatic mechanism would come from novels and intuitions, Mitchell et al. would be right that science should ignore it. But we have shown good reasons to believe that the automatic mechanism must still be present in humans. Perhaps the problem is that the Proust effect has always been taken for granted and proofs have not been searched in the right places.