Working memory
The brain's desk. About four things fit on it. Everything else falls off or has to be written down.
Working memory is what you're holding in mind right now: the phone number, the thing you were about to say, the three steps of the task. It's small, roughly four items for most adults, and it empties when something else grabs it. Evidence: solid
It's also one of the first things stress and tiredness hit. That's why on a bad day you lose the thread halfway through a sentence. Evidence: decent
Why it matters
Trying to hold your whole week in your head is running a desk with fifty things on it. Lists exist because the desk is small. So do closed doors and phones in other rooms. Evidence: decent