Grain of salt

A sort of proverb, the Latin expression indicated that in everything we should use at least a grain of the salt with which the head was, in Roman times, presumed to be filled. It was, then, a recall to common sense, a request for prudence or reflection before action.

To take something "with a grain of salt" now means to accept it less than fully. The Oxford English Dictionary dates this usage back to 1647.