It is a fascinating quirk of human nature that a major portion of our mental activity occurs at a level inaccessible to our conscious selves. Freud distinguished between the conscious, unconscious, and preconscious; the Buddha characterised the unconscious as an unruly grey elephant and the conscious mind as its rider; and Dr David Lewis divides our mental activity into "System I" (for "impulsive") and "System R" (for "reflection") thinking.
System I thinking uses fixed mental sub-routines called heuristics in order to quickly categorise incoming sensory data and then respond accordingly. It is emotional and instinctual; it creates "gut feelings" and first impressions. It is also easily fooled and, as savvy marketeers and politicians know, easily manipulated.
System R thinking is rational and reflective, and gives us restraint and forward-planning as well as such wonders as literature. But what dozens of varied examples in Lewis's book all illustrate is that System I thinking – "our zombie brain's default mode" – takes precedence over System-R thinking, and is responsible for the majority of our everyday behaviours. System R thinking comes into play after the fact: "Only if asked why we spoke or acted as we did, do we engage System R thinking to try to come up with a sensible, or at least plausible explanation for our behaviour ... it serves as a PR department for the 'self', explaining, embellishing and justifying our actions."
Doubtless it is worthwhile, however disconcerting, to be mindful of the fact that we aren't as rational as we appear even to ourselves. It helps explain human failings from prejudice to addiction. Luckily, we live in an era in which the general reader attracted to the explanatory power of modern neuroscience is spoilt for choice. And Lewis is neither the first nor the most elegant writer to have described the same dichotomy.
Lewis gathers his supporting evidence from sources ranging from the social sciences' most seminal experiments, to a study he conducted himself on the Channel 4 programme Secret Eaters. But it isn't clear how much credence he expects us to apportion to each study, and it begins to seem as if every scrap of information, no matter how anecdotal, is grist to his mill. And the speed with which he skates over the surface of things can be disorienting.
Some of the book's more interesting anecdotes come from the world of marketing, where millions of pounds are spent researching ways to appeal directly to our System I brains. Indeed, Lewis is described by his own book's blurb as the "father of neuromarketing". Perhaps he has indirectly caused you to spend more money in a supermarket than you otherwise would have. If so, don't feel too bad if, this time, you do engage System R, and interrogate the impulse to buy his book.