By “mental development” and “leaps in mental development” we mean the natural growth of the brain and the entire nervous system from a few neurons making connections in the embryo, all the way to the fully functioning brain and nervous system of an adult.

Just like other organs in the body, the brain grows, adds new functionality, and matures. As it does (in stages, on nature’s schedule) the cranium grows, neurons are added, and successively higher perceptual functions emerge.

van de Rijt and Plooij identified ten such periods in the first 20 months of life, during what Piaget called the “Sensorimotor period”, the period when as developmental psychologist J.W. Santrock put it: “An infant progresses from reflexive, instinctual action at birth to the beginning of symbolic thought toward the end of the stage.”