" In fact, we’re not smart and we have bodies—we’re smart because we have bodies.7 The heart has about 40,000 neurons that play a central role in shaping emotion, perception, and decision making. The stomach and intestines complete this network, containing more than 500 million nerve cells, 100 million neurons, 30 different neurotransmitters, and 90 percent of the body’s supply of serotonin (one of the major neurochemicals responsible for mood and well-being). This “second brain,” as scientists have dubbed it, lends some empirical support to the persistent notion of gut instinct. "

― Steven Kotler , Stealing Fire: How Silicon Valley, the Navy SEALs, and Maverick Scientists Are Revolutionizing the Way We Live and Work