@gozz,
I have also heard that honesty is the best policy. If you translate the Norwegian equivalent of that saying directly to English, you get "Honesty last the longest", to which the most common reply is "because it is the least used".
But it is hard to know what Descartes really meant based on one sentence. As I read it, my immediate thought is that it sounds counter intuitive. Many people lack the good sense to pour water out of a boot.