A spare key matters because an ordinary lapse should not have to become a crisis.

Good systems assume people will be human. Everyone will at some point leave the keys inside, forget the password, miss the train, and the question is only whether the arrangement around them turns a five-minute lapse into a lost evening and a locksmith’s bill. Redundancy is not pessimism about yourself; it is an accurate estimate of ordinary life. The people who seem to have fewer disasters usually have more backups.

Spend an hour this week on the boring provisions: a key with a neighbour, the documents copied, the number written somewhere that is not your phone. Tedious now, and quietly generous to your future self.