Si users rely on Experience. They follow instructions exactly, enjoy their comfort zone and what they've already experienced, compares new information to the old, and will always be prepared when going into the unknown. Once he makes a mistake, it can almost be guaranteed he won't make it again. He is traditional, cautious, steadfast.
Ni users rely on Intuition. They perceive universal truths eg. "what goes around comes around" and "do to others as you would like to be treated." They are naturally aware of and listen to their conscience, see "visions/premonitions" of the future, take nothing as what it may seem on the surface but at it's greatest depths, sometimes just "know."
Se users rely on their Instincts, Ne users their Insight.