A lot of historians throw around the term ‘agricultural surplus.’ By this they mean food which the farmers and their livestock don’t eat, and which can be used to feed stonemastons and metalworkers and scribes and priests and gentlemen farmers. In this theory, societies have to find a way to produce a larger surplus before they can produce things historians like such as books. I think this term is one of the terms which historians borrowed from economists in the early 20th century.
At first the idea seems harmless enough: if a family needs 20 bushels of barley to feed itself and its animals and have seed for next year, and they harvest 30, they will probably trade 10 for something else or use it to fatten stock. But in the real world there is rent and taxation. And when you look at the science of nutrition, you find that there is a range in the amount of food that farm workers eat. At the low end, they can’t work very well, lose most of their children, and die young of chronic diseases or infections which their weakened body can’t fight off; at the high end, they have a varied diet, grow taller and stronger, and can be pretty sure of having surviving children. Its not actually the case that people need a certain number of calories of Generic Food ™ a day, above which they just get fat and below which they die. Taxes and rents often come out of this margin in between. And it is usually taxes and rents which pay for the stone buildings, the scholars writing treatises on ethics, and the beautiful silver cups.