Chapter One
A Balance Is a Photograph
At the grocer's counter, a person reaches for a phone before choosing whether to add a small treat to the basket. The account screen shows enough. They put the treat in. At home, they remember the rent transfer that has been arranged but has not appeared on the screen. The treat was hardly extravagant. The difficulty was that the screen answered a narrower question than the one they meant to ask.
An account balance is a photograph of an account according to its provider's rules at a moment in time. It is not a photograph of the next week. It may also be different from the amount you can actually use while transactions are pending. Even if every entry on the screen is perfectly accurate, it cannot include an obligation the account does not know about. That might be a bill waiting in an email, a renewal tucked in a drawer or a promise made to another person.
This is why the sentence “I have enough” needs an ending. Enough for this purchase alone? Enough after the money due before the next pay arrives? Enough if a card payment is taken sooner than expected? The phone can help with the first question. It needs your calendar to help with the others.
Imagine an invented household with a balance of 900 units on Friday. On Monday, rent of 500 units is due. On Tuesday, a card payment of 180 units is due. Food and travel until the next pay are expected to cost 150 units. The account is not empty, but only 70 units are unassigned if all those expectations hold. The arithmetic is deliberately plain. The hard part is remembering that the four numbers belong on the same page before Friday's balance becomes Friday's decision.
