Think of the non-living elements of nature, then consider all the behaviors they can collectively exhibit: wind, clouds, fog, rain, frost, snow, sleet, hail, lightning, thunder – and these are just the atmospheric elements, air and water. Now imagine what behaviors would naturally arise in a large system of widely interconnected, complex elements – say the world’s populace or your brain.
Each of the 10^11 (one hundred billion) neurons [in the human brain] has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10^15 synapses (1 quadrillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 10^14 to 5 x 10^14 synapses (100 to 500 trillion).