Ask someone to name a famous psychologist, and chances are they will pick Sigmund Freud. His ideas about the unconscious—a sort of shadowy basement of the mind that is inaccessible to rational thought, but which nevertheless influences people’s behaviour—are part of popular folklore. Although it remained popular at dinner parties, the idea of the unconscious fell out of favour among 20th-century psychologists, thanks to the rise of more scientific approaches to psychology. These focused purely on studying behaviour and refrained from theorising about the inner workings of the mind. In his latest book, "Subliminal", Leonard Mlodinow, a theoretical physicist, shows how the idea of the unconscious has become respectable again over the past couple of decades. This development has been helped by rigorous experimental evidence of the effects of the subconscious and, especially, by real-time brain-scanning technology that allows researchers to examine what is going on in their subjects’ heads. That experimental evidence suggests that, as Freud suspected, conscious reasoning makes up a comparatively small part. of the activity in our brains, with most of the work taking place where we can’t tap into it. However, unlike Freud’s unconscious, the modern unconscious is a place of super-fast data processing, useful survival mechanisms and rules of thumb about the world that have been trained by millions of years of evolution. It is the unconscious, for instance, that stitches together data on colour, shape, movement and perspective to create the sight enjoyed by the conscious part. of the mind. The modem view of the unconscious mind may be more benign than Freud’s, but it can still generate unwelcome impulses. Psychologists theorise that the well-documented tendency of humans to categorise almost every piece of information they come across is a survival mechanism that evolved to aid quick decision making. Yet it may also lie behind the tendency for human beings to group people into races, genders, creeds and the like, and then to apply certain characteristics—unjustifiably—to every member of that group. The insights offered by modem science into the workings of the human mind are fascinating in their own right. But they also suggest that plenty of conventional wisdom about how humans behave may need rethinking. For instance, Mr Mlodinow notes that economic models are built "on the assumption that people make decisions by consciously weighing the relevant factors", whereas the psychological research suggests that, most of the time, they do no such thing. Instead, they act on the basis of simple, unconscious rules that can sometimes produce completely irrational results.