There are several 'life lessons' that I've picked up from playing bridge. Some of them are incredibly obvious in hindsight. Some of them less so. This is one of the more obvious in hindsight, but also one of the more important, so I think I'll start with it.
It is fairly common when playing bridge to reason thusly: in order to make this contract, I need West to hold the King of Diamonds. If West holds the King of Diamonds, then he can't hold the Ace of Spades. Therefore, I'll assume East holds the Ace of Spades and play accordingly (an example).
There are a surprising number of applications of this principle to real life.
In order for me to have any chance of getting this job, they would have to be willing to accept someone with my level of qualifications, so I will assume that they are willing to accept someone with my level of qualifications, and act accordingly.
In order for us to have any chance of treating the patient, they must have disease X rather than disease Y, as disease Y is untreatable, so I will assume that they have disease X, and act accordingly.
In order for their to be any chance of my making it on time for my very important meeting, there will need to be a train waiting at the station at the second I get there, so I will assume there will be such a train and act accordingly.
In order for there to be any chance of me going on a date with this girl, she would need to be the sort of girl who responds positively to my asking her out. Therefore, I'll assume that she will respond positively, and act accordingly.
There is apparently an old Yiddish Proverb which reads "if you've got nothing to lose, you can try everything". This is a specific case of the general principle at work here. The serenity prayer which I bastardised for the title of this post gets a bit closer "Through my efforts, I gain the serenity to accept the things I cannot change; courage to change the things I can; and the wisdom to know the difference.", but doesn't seem to get the emphasis quite right.
There must be a proverb that sums up this insight... anyone?