How Credible Is the Premise?
You receive an unsolicited e-mail from Dr. Samuel Jones III, who says he is a noted attorney from some country you've never heard of. The e-mail reads, “The late King of the eastern region has died and bequeathed to you $50 million. For me to send you this money, please send me your social security number, with two credit card numbers (don't forget the three-digit CCV code on the back). Looking forward to your response.”
Most people would conclude this is a bogus e-mail and delete it. They do so because the premise's observations have almost no credibility. Their brains say, “What's the probability that any of these observations is true?” Much has been written about these schemes and credit card fraud, so they assume that this is one of those scams and delete the e-mail.
On the other hand, you might be listening to the radio, and the emergency broadcast system interrupts and warns of a tornado in your county. You look outside at some menacing clouds, so you conclude that you should head to the basement. Why? Well, the emergency broadcast system isn't used to spread rumors, and it says a tornado was spotted (observation); you see menacing clouds outside—an experience that gives credibility to the alert—and you have multiple observations from tornado stories. These components lead you to assume a tornado is probable, so you hightail it to your basement.
You can see how important premise credibility is.