There are two ways to think about the inverse relation between adding and subtracting. One is in terms of identity: If I add some stuff to an object and then subtract the same same stuff from that object, then the object will be as it was before I added and subtracted anything from it. The other way of thinking about the inverse relation is quantitative. If I  add a number of things to a set and then subtract the same number of things from that set,  the original quantity is now the same as it was at the start.  I will present evidence of a developmental progression in young children from understanding the inversion of identity to understanding the inversion of quantity.