86 billion neurons, and they learn by repetition

Your brain has about 86 billion nerve cells, called neurons. In 1949 the psychologist Donald Hebb suggested a simple rule that is often summed up as "neurons that fire together, wire together". Each time a pattern of thought repeats, the connections that carry it get stronger.

That's why a voice you have heard hundreds of times, like "you need top marks" or "you're not good enough", can feel automatic. It has worn a deep path. The good news is that the same rule works in reverse: each time you stop and ask "whose voice is that?", you practise a new path.

Sources: Azevedo et al., Journal of Comparative Neurology, 2009; Hebb, The Organization of Behavior, 1949.