Re: Ch. 2: The Intuitive Dog and Its Rational Tail
Maybe Hume intuitively understood that we make snap judgments based on emotion—before reason can kick in. And to pretend that reason rules the day is delusion. Maybe that's where the "ought" comes in. We ought not pretend that we are purely rational beings.
Indeed, here's a more complete version of the same quote:
"Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them."
But it would be better to get it straight from the horse's mouth. This is Hume's Treatise on Human Nature
I think Haidt will say that reason can sway the elephant at least in some circumstances.