Allswell is a chemical that naturally occurs in the human brain (at least, the humans of Arbre). It makes people feel a sense of well-being and optimism. Some centuries ago, it was genetically engineered into the plant named Blithe, which is a dietary staple throughout Arbre.
Throughout Arbre except in the maths, that is. The avout are forbidden to consume it – or even to grow it – and their time spent outside their maths is limited to ten days in order to stop it from starting to affect them adversely.