> You can't have -5 rocks. Negative numbers are just the solution to equations of the form x + n = 0. The best mental model of negative numbers that works for

> You can't have -5 rocks. Negative numbers are just the solution to equations of the form x + n = 0. The best mental model of negative numbers that works for me is to treat it as direction. In your example, if I have "-5" rocks it means I owe +5 rocks to someone else, let's say John. If, after a few days Rachel were to give me +5 rocks and then you square it off with John you are left with no rocks. Directionally Rachel → (5 rocks) You → (5 rocks) John; So +/- stand for things flowing into and away from you respectively.