A doctor, a lawyer and a mathematician were discussing the relative merits of having a wife or a mistress.
“For sure a mistress is better,” says the lawyer. “If you have a wife and want a divorce, it causes all sorts of legal problems.”
“No, no, it’s better to have a wife,” says the doctor, “because the sense of security lowers your stress and is good for your health.
“No, no, you’re both wrong,” replies the mathmatician. “It’s best to have both so that when the wife thinks you’re with the mistress and the mistress thinks you’re with your wife, you can slip away and do some mathematics.”