Do not believe in anything simply because you have heard it.
Do not believe in anything because it is spoken and rumored by many.
Do not believe in anything simply because it is found written in your religious books.
Do not believe in anything merely on the authority of your teachers and elders.
Do not believe in traditions because they have been handed down for many generations.
But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it.
via Of Petrified Giants and the Credulity Crisis | Firedoglake. (h/t to Glenn W. Smith, who recounted it as part of his excellent post on credulity, the quote attributed to Buddha in the Kalamasutra)