You are eating rock. That is not a metaphor. Table salt — sodium chloride, NaCl — is a crystalline mineral, geologically identical to the rocks beneath your feet, and you consume it every single day. Mark Kurlansky, whose landmark history of salt traces its role across six thousand years of civilization, argues that no substance has shaped human society more quietly or more completely. Salt production dates to around 6050 BC, when early peoples evaporated seawater to collect it. That is older than the wheel. Last lecture, we established that a bee colony operates like a brain — simple local rules producing collective intelligence that underwrites the food supply of the entire planet. Salt runs a parallel story, except its intelligence is chemical, not biological. Your body is roughly 0.4 percent salt by weight, and that fraction is non-negotiable. Sodium ions regulate nerve impulses, muscle contractions, and fluid balance at the cellular level. Without it, neurons cannot fire. The heart cannot beat. Salt is not a seasoning. It is infrastructure. The ancient world understood salt's power long before biochemistry named it. Around 2700 BC, the earliest known Chinese pharmacology text documented over 40 types of salt and their extraction methods. Ancient Egyptians used it for mummification and religious offerings by 3000 BCE, and Phoenicians traded it across the entire Mediterranean. Rome formalized the obsession: soldiers received a salarium — a salt allowance — as part of their pay in the 4th century BCE. That word is the direct ancestor of the English word salary. You are still being paid in salt. You just don't know it. Salt didn't just pay soldiers — it built empires and toppled them. Venice rose to medieval dominance through a salt monopoly, controlling supply across Europe. In ancient Ethiopia, rock salt slabs called amôlés served as physical currency. In 1295, Marco Polo described salt coins sealed by the Khan in Cathay. Salt revenue financed construction of the Great Wall of China in 119 BC. France's gabelle — a salt tax — generated such resentment it became a direct accelerant of the French Revolution. The Dutch blockade of Iberian salt works contributed to Spanish bankruptcy in the 16th century. One mineral. Entire geopolitical consequences. Salt also preserved civilization literally, not just economically. During the Middle Ages, it was the only reliable method of keeping food through winter — no salt meant starvation by February. The word salad comes from the medieval Latin salata, meaning salted, because vegetables were preserved in brine. Salt routes crisscrossed the globe, from Morocco to Timbuktu, Egypt to Greece. Sanctuary, here is what this course keeps returning to: the most powerful forces in history are rarely the ones that announce themselves. A pinch of mineral on your table carries the weight of six thousand years of war, trade, survival, and cellular biology. Common things carry extraordinary histories. You just have to look at what you're actually holding.